Penalty Cards: These are the OP cards that the UR staff has put penalties on. Basically, if you win with them in your hand, you have to face a penalty. I won't list the penalty cards, but generally, it's better not to use them. There are -2 penalty cards, -5 penalty cards, and -10 penalty cards. You can MAYBE use one game-changing -2 penalty card, but -5 and -10 cards are OFF LIMITS.
Other cards you should stay away from: DRs, Pill Manip, poison, heal, +life
DRs: They slow down the game. In DTs, you need to play FAST. DRs may help you win, but it's usually better not to use them as it stalls time. You MAYBE might use one, but that's it. This is the reason why Pussycats
can't be played in DT, because they play too slowly. In Class 2, it is better to drop the DRs and just get 2HKOers, and in Class 1, the dmg is already low because the *s are low.
Pill Manip and +life: Not as much as DR's but pill manips and +life can also slow down the game. Your opponent may go like 'Oh mah goodness, is he going for it, is he bluffing, or is he anti-anti-bluffing, or anti-anti-anti-anti-bluffing or is he...' and take forever to play. Don't use excessive amounts of these cards, but a little is okay.
Heal and Poison: The problem with poison and heal: They take time to activate. In Class 2/3 where it's all 2HKO's, this is a problem. They also both somewhat take some time to think about, which will slow the game down. U might consider them in class 1, but in the other classes, it's not recommended.