Backstab: Basically, if some level of surprise or flanking is achieved by the character, they can increase the amount of damage they do. I've seen different mechanics for this, and I don't have an objection to this, I don't think. It does need to be limited, though; none of this "I'm flanking for the eighth combat round in a row so I still get to backstab" nonsense. I figure letting a Scoundrel that chooses this as an ability double their damage during a surprise round. When the surprise round is over, they're back to normal damage. Practically speaking, this will only be a good ability for a Scoundrel to have if they also put resources into being stealthy, making this ability almost cost double. I say that if a player wants to have his Scoundrel backstab badly enough to invest in both backstabbing and being stealthy, let him have his backstab. Verdict: Ability
Assassination: Basically, if your character surprises someone, they get a chance, based off of 50%, with adjustments based on your character's level and your character's targets' level, to flat out kill the target, with massive damage resulting if your character fails. All of this automatically, with no roll to hit. Raiiiiiiiiight. That's a nice talent for the Fifth Circle of a Glantri style Assassin prestige class, maybe, but not something I want available outside of that kind of situation. Verdict: Out
Two-Handed Fighting: I've seen various mechanics for this, and don't have a problem with this either. I don't see why it should be reserved for Scoundrels, though. A fighter should be able to do this, and I don't really see a problem with a two-dagger-wielding magic-user either, for that matter. I'd put this in combat rules, not skills or thiefly abilities. Verdict: Out
Poison Use: Basically, a skill that keeps your character from accidentally poisoning himself, and also allows him to identify poisons. I'm willing to grant that PCs are generally competent enough to not poison themselves, generally speaking, and bundle identifying poisons in with poison brewing. Verdict: Out
Poison Brewing: Brewing poisons and their antidotes. I'm inclined to relegate this to an alchemist and/or assassin prestige class, though I'm more on the fence with this one than I have been with most of these other skill/ability candidates. Verdict: Out
Think this is letting Scoundrels be too powerful in combat? Not powerful enough? Think poison should be more available to players? How do you use poison in your games, anyway? Let me know!