It would be great if someone could shed some light on how Obsidian plans to incorporate this aspect in the game.
I always roll a thief/assassin character in cRPGs and I am curious how it will turn out to be in PE.
IMHO, BG2 had a very satisfying feel for the assassin - picking the right tools, items and skills, you could one-shot very hard foes before they got a chance to react. After that, you were kind of useless unless you had invisibility potions or items to do another stab, or it was running in the fog to re-stealth.
The principle was very good and mostly satisfying but there were some pitfalls PE could learn from - first off, the one that most developers fail at - backstabing being based on weapon base damage / all weapons are equally good at stabbing. This must be avoided as a Staff of the Ram with huge base damage is an optimal backstabbing weapon no matter what.
Instead, there should be a hefty bonus for daggers/short swords to make them the ideal weapon for such things. Dual-wielding should also amplify the backstabbing capabilities but should also have drawbacks compared to one-hand attacks.
That said, a mace/sword rogue should definitely be a viable path and could have more consistent damage during combat as opposed to daggers that do a big initial burst but then perform worse as the clash stretches out.
Another very important part is stealth and the mechanics surrounding it. I believe it's a consensus that the visual/audio hiding/checking should be merged into one, as no one would max one and leave the other low because that would still make sneaking impossible. Trying to go into stealth and "failing" was also mind boggling during the stages where your sneaking wasn't that good, requiring you to just sit there a couple of turns until you finally enter successful, so I think every attempt should be successful, it's just that lower levels would require you to be slower and more prone to detection.
Visual cones is something that should definitely affect sneaking, as should walls and items that block vision (like in BG2). It shouldn't be too complicated though, a simple flat detection bonus should be applied if you are sneaking in front of someone as opposed to being behind them. Naturally, distance would factor in a lot as well.
Anyway, I'll add more thoughts tomorrow, but feel free to add in or comment