I'm just getting into Two-Headed-Giant games, and have noticed that some cards become far more powerful while others become much less powerful. For instance, landwalk cards become better because your opponents control more land, mass damage prevention cards like fog improve because your opponents attack with more creatures, and cards that require a longer amount of time to work better improve because the matches tend to last a greater amount of turns. On the other hand, weanies become slightly weaker because it's more likely your opponent has at least a few blockers than can kill them off, and cards that counteract a specific color tend to weaken because your opponents are very likely to have other colors that are immune to its effects.
What cards or types of cards become significantly better or worse in THG than in 'normal' magic? How can I recognize a card that will be strongly affected by the switch to THG rules? What factors of THG tend to cause these changes in usefulness?