The Block Constructed season is on us in full force. A host of Pro Tour Qualifiers and Grand Prix events will be run over the next several months using this format. Herewith is coverage of some of the difficulties you're likely to see, both with card interactions and with the mechanics of play.
Kicker: Some Kicker abilities (like the black version on Stormscape Battlemage or the red version of Thornscape Battlemage) generate coming-into-play triggered abilities. These abilities are the color of the source that generated them (blue and green, respectively). Therefore, the Thornscape's ability can damage a creature with Protection from Red. Regardless of the colors in the Kicker cost, the mana cost of the spell is still what's in the upper right-hand corner of the card (so the Stormscape is still 2U). Confound cannot counter a Battlemage, because the spell doesn't target a creature-the triggered ability does. Finally, the only time you can pay the Kicker cost is when you're playing the spell from your hand. If you put the card into play some other way (like Dragon Arch or Death), you don't have the opportunity to pay the Kicker.
Gating: The creature to be Gated is chosen on during resolution of the triggered ability. Gating does not target.
Token/Animated Creatures: They only have a creature type if the ability says so (Goblin Trenches), not if it doesn't (Life).
Split cards: Anywhere but on the stack, they're both colors. If something is trying to match a mana cost value (like Void), it will match either. If something is simply looking for a value (Planeswalker's Fury), it will add the total of both mana costs.
Choices: Choices, other than targets and modes, are made on resolution, not announcement. Some cards that fall into this category are Addle and Wash Out.
Paying the kicker on these cards generates a coming-into-play triggered ability
Blurred Mongoose: Just like Urza's Rage, it can be targeted by counterspells; they just don't do anything to it. The Mongoose's untargetability is only when it's in play.
Charms (and other modal spells) and Flagbearers. Modes are chosen before targets (409.1b vs. 409.1c). That means you can choose a mode that won't require you to target the Flagbearer.
Dodecapod: This is a replacement ability, not a triggered ability. There's no way to remove Dodecapod from the graveyard when it's discarded (such as Cremate), because it never gets there.
Fact or Fiction: The Fact or Fiction goes in the graveyard on top of what cards weren't chosen to go in the player's hand. No one may play spells in the middle of resolving Fact or Fiction.
Familiars: A given Familiar can only pay 1 toward the cost of playing a spell, even if the spell has both the listed colors. For example, with a Nightscape Familiar in play, you'll pay 2UR for a Prophetic Bolt. Familiars help with the total cost to play the spell, but again only 1. You don't get 1 for the mana cost and 1 for the Kicker.
Gainsay: Since it says "counter target blue spell," the spell must be blue before Gainsay targets it. Same goes for Slay; the creature must be green before it's targeted.
Mask of Intolerance: It will only trigger at beginning of upkeep if its condition is met; if so, it goes on the stack. It will check again on resolution; if the condition is still true, it has its effect.
Mystic Snake: You may play it as an instant even if there's no spell on the stack. The counterspell part is a coming-into-play triggered ability, and does not require choosing targets until the triggered ability goes on the stack.
Orim's Thunder: If played with Kicker, it doesn't matter what zone the enchantment is in when the Thunder resolves; its converted mana cost is still the same. A Kicked Thunder is only countered if both the enchantment and the creature are illegal/invalid on resolution.
A Sanctuary's effect is determined when its ability resolves.
Sanctuaries: Players and Judges continue to get tripped up by this. Each Sanctuary has an ability that triggers only if its controller controls a permanent of one of the enchantment's enemy colors (for example, Dega Sanctuary is white and triggers on control of a black or red permanent). If the player doesn't control a permanent of the right color at the beginning of upkeep, the ability never triggers in the first place; he can't change the color of a permanent during upkeep to make a Sanctuary's ability trigger.
If the player controls a permanent of *one* of the enchantment's enemy colors when the ability triggers, and then changes the color of another permanent to the other enemy color before the ability resolves, he'll get the bigger effect. If he controls a single permanent that's both of the enchantment's enemy colors, he'll get the bigger effect. If he controls no permanents of the Sanctuary's enemy colors when the ability resolves, it will have no effect.
Unnatural Selection/Pure Reflection: With Pure Reflection in play, changing creatures to type "Reflection" will cause Pure Reflection's triggered ability to destroy them. They can still be regenerated.
Yawgmoth's Agenda: It will remove itself from the game if destroyed. If you play your own spell to destroy it, the Agenda, but not the spell that destroyed it, will be removed. That's because putting a spell card into the graveyard is the LAST thing that happens during resolution (413.2h); by the time it does, the Yawgmoth's Agenda (and its effect) is already gone.