YOU (20 life): in play - Plains x2, Suntail Hawk (equipped with Bonesplitter); in hand - Plains, Diving Griffin, Samurai of the Pale Curtain, Leonin Skyhunter, Terashi's Grasp.
OPP: in play - Urza's Mine, Urza's Power Plant, Talisman of Impulse
This turn will say a lot about your entire game plan. Your two choices are to play a creature or to play Terashi's Grasp.
If you play a creature, which would be Diving Griffin in order to use your mana efficiently, you're going to use Terashi's Grasp later on to kill an Oblivion Stone or other threat if you are given the chance. In exchange, you've declined to go after his mana. He will soon have both colored mana and colorless mana, so you'd better hope that nothing too crazy happens to you. Sometimes you'll be able to stop his only threat with Terashi's Grasp and he'll choke on mana sources.
If you kill the Talisman, you're trying to do one of two things. By killing Talisman you've removed the only source of green (and red) that you've seen so far, so there's a chance that your opponent does not have another one. Even if he does, there is a very good chance that he will lose a turn of mana development, pushing his spells back a turn. In this case, you have bought valuable time as your opponent is already taking three damage per turn. If all goes according to plan he'll go down to fourteen this turn, eleven next turn, six the next and die on turn six. Also note that playing another creature now can speed up your clock, but only if you draw a Plains next turn or another card that speeds up your clock: Diving Griffin will knock him to nine on turn four, then your current hand can only allow you to do seven more damage on turn five.
Also note that in each case you can kill him on turn six and do all your damage in the air, so you don't have to worry much about a blocker. If he had a Birds of Paradise, it would be on the table right now.
Ah, that's the other big problem with waiting for the Oblivion Stone. A smart opponent is unlikely to give you a chance to destroy the Oblivion Stone. It costs eight mana to play the Stone and use it on the spot, and the Urzatron provides seven mana. He already has two of the three pieces of the Urza's set, and he's about to untap with green mana to use all the spells that let him find Urza's Tower. To make matters worse, he could already be holding Urza's Tower, especially since he would have played the lands in this order if he did have all three (remember this lesson?). There's a good chance he will spend turn three setting up the Urzatron, or holding back to collect more collateral damage, and then clearing the board on turn four without giving you a chance to stop him with Terashi's Grasp.
Now to ask the classic early-turn question: What do you know about his hand? What is the chance he has another source of green mana?
He could have another green source without having to have made a mistake. If he has Urza's Tower, he could be trying to get all three lands online on the third turn and therefore not be interested in playing Forests. He could also be putting out the Talisman as bait. This is the big worry for you: If he can play Talisman and get you to play Grasp, he's traded his two mana for your three mana and dealt with a card that could potentially prevent him from saving himself with Oblivion Stone. Remember that you can't kill until turn six, giving him time to use the Stone on turn five without any heroics from the Urzatron. So there is the risk that this is a trap. The other reason to be suspicious is that your opponent did not have to play the Talisman this turn. If the mana is life or death, and he had a third colorless land, he would hold the Talisman if he was going to try and play Sakura-Tribe Elder or Sylvan Scrying on the same turn with the mana. The only ways he has to expose a vital mana source here are if he is going for Reap and Sow… or he doesn't have a third land. However, these are also the situations where you can knock him right out of the game with Terashi's Grasp.
Also note that your opponent's Oblivion Stones will make him reluctant to play a Talisman he could have held, so he had a reason to play it. Either it is bait or he needs it.
OK, now it's time to add all of that up. Should you take out the Talisman?
Yes. The waiting plan plays into your opponent's biggest strength, which are his expensive long-term spells. He even has Eternal Witness to reclaim cards you have killed. The only games in which this will win you the game are those in which he is trapping you with the Talisman in order to allow Oblivion Stone to survive. This is not crazy, since he knows that you play the card, but your opponent likely has at least thirteen cards that allow him to shrug off that Grasp and still use a Stone: Sylvan Scrying, Reap and Sow, Urza's Tower… and extra copies of Oblivion Stone. It could easily be a higher number, but already that makes the chances of stealing a win in this way rather slim. On the other hand, there is a reasonably high chance that he has no other green source right now and won't be topdecking a Forest. While there is a chance the Talisman is bait, there is a much better chance it is not. A conservative estimation would be that 50% of the time your opponent has no green source or needs to cast Reap and Sow next turn, 25% of the time he wants you to kill the Talisman and 25% of the time he is fine with either decision. However, you could also judge your opponent to be relatively unskilled due to his willingness to play Talisman and four copies of Oblivion Stone when there are better mana configurations, allowing you to discount the chance of a trap.
The game played out as follows: The Talisman was not killed, and it was the only green source his opponent had, allowing him to play a Sakura-Tribe Elder. While his opponent did in fact expose and lose an Oblivion Stone to the Terashi's Grasp, the Tooth and Nail player managed to win the game by a turn. That raises one last note: If you do let the Talisman live, your opponent will likely discount the chance that you are holding Terashi's Grasp.
A second situation comes from an Eighth Edition draft. Thanks for this one go out to Jim Mason. Rather than pose the original puzzle he did, I'm going to back up a little. We don't know whether the opponent mulliganed, so you'll need to consider both scenarios if they are different:
It is your third turn. You just played a land. What spell do you cast?
That question wasn't entirely fair to those who don't know 888 draft, but I felt it was important not to give away important information right away. The first thing you should ask is: How likely is it that your opponent has spells for two mana that can get him more mana? The answer is that both Vine Trellis and Rampant Growth are common cards that are likely to make it into the deck of any green mage who has drafted them.
What is the chance he has one of those two cards in his hand? Both are common and worth playing, so there's a very good chance there are one or two such cards in his deck. However, we don't have to rely on random chance here, because our opponent has already made one big decision in this game. The first thing every player does is choose whether to mulligan. Your opponent chose to keep his hand. Why did he do that? If he kept a seven card hand with only one land in it, chances are extremely high that it included another mana source like Rampant Growth. The better your opponent, the more you can rely on that logic, because some poor players won't always realize they need to mulligan a one land hand.
If he did not mulligan, you can reliably put him on Rampant Growth or Vine Trellis. If he did mulligan, that becomes much harder, because even a bad six card hand is often worth keeping. It is better to need to draw three lands than to have so few cards that even a good draw won't save you.
If he has one of those two key cards in his hand, you have a huge incentive to try and knock it out of his hand before he finds his second land. This will cost him a mana that he needs badly, slowing down his development by a minimum of a turn even if he draws the land he needs as soon as possible. If he has to dig for it, it could cost him a lot more than one turn. This clearly outweighs the extra six damage you can get in by casting the Serpent Warrior a turn earlier. Even if you don't find what you are looking for, you can still remove his best two drop or the creature that can best stop your Serpent Warrior. If Coercion goes to waste, you are probably going to pull this one out either way: He has no mana sources and he's lost his most relevant spell, so he's going to have a hard time winning.
This story also has an unhappy ending: Coercion was cast and it nailed Rampant Growth, but the opponent, which is defined in the glossary as a sly individual with an ugly face, drew lands on each of the next three turns and went on to win the game. That was unfortunate, but it doesn't change the fact that Jim's play was definitely the correct one.
That would make it even more imperative to hit your opponent's hand before he draws his second land, but it can cut both ways. Your chances of winning the game if your opponent draws out of his mana problems have gotten a lot smaller, so you need to play to take advantage of those mana problems. That adds to the need to hit his land. The problem is that Coercion could now be time you can't afford to lose: If he draws the second land next turn, things are already so bad that you are unlikely to win in any case. In that case, you can cast the Warrior first to speed up your clock by three points and gamble that he won't draw a land. The worse your long term outlook, the bigger the chances you have to take. I don't think you should do that, but it is worth considering.
Nothing in Magic is free. When you have a great late game, it tends to come at the cost of a vulnerable early game. When you have a strong early game, your late game will suffer. This applies both to decks and to games. When I put a Kokusho into my deck, I'm helping my late game at the cost of my early game. When I put in a Suntail Hawk, I'm doing the opposite. The same thing happens with opening hands. If I start with a lot of lands and cheap cards, I have a strong early game but I'm going to run out of ammunition if the game goes long. I need to push that advantage. If I don't draw enough lands, or I don't draw any cheap spells, that makes me highly vulnerable now, but my opponent needs to take advantage of that. If he does not kill me, those cards I have will make me stronger. Later in the game, I'll have more powerful spells and bigger creatures while my opponent will be choking on useless lands.
If your advantage lies early, play to win the game early. That applies to trying to stall your opponent's mana development and to other aspects of the game as well. You need to press your attack to get maximum damage in, to cast your spells before they can be countered. Those early turns are when you can win the game, and you probably won't get another chance.