What cards or types of cards are substantially more/less useful in Two-Headed-Giant than in 'normal' Magic?

Question

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?

Answer 1

Any cards that care about any aspect of the game that is likely to increase due to there being more Lands, Cards in Hand, Creatures in Play, Higher Life Totals, turns, attack phases, etc.

The above is just a generalization of course. THG is similar to multiplayer in this respect. Aggro decks don't work as well, and Combo and Control decks get better. Some interesting examples are listed below, but in general you should be looking at cards that count things that there are likely to be more of. Truthfully though, most THG decks are built as two puzzle pieces (unless playing draft/sealed). Cards are specifically chosen to complement the other deck. While generalities are useful, they aren't the whole picture.

Life Totals start at 30, and are shared by both players. Any card that counts Life Totals will behave differently. Some interesting examples are:

Heartless Hidetsugu - Instantly kill all even life teams.

Serra Ascendant - Starts off with +5/+5 bonus.

There will be more lands in play, interesting examples are:

Impending Disaster - Almost sure to trigger, and cheaper than Armageddon.

Limited Resources - Broken for Multiplayer, and bad in THG with decks built to exploit it.

There will be more Hands. Interesting examples are:

Unnerve - 1:4 your opponents isn't bad.

Answer 2

Some things that are a bit broken in Two-Headed Giant are things that add phases (e.g. time walk or extra draw steps, or extra attack phases), or do the opposite, like cards that skip phases (e.g. skip your draw step). This is because these effects are effectively doubled.