Recently I've been playing Iji. A free 2D platform shooter/rpg for the PC. It's fairly unique in it's genre providing more detailed story and leveling system than your average 2D shooter. It's been in production for four years and the attention to detail really shows. From well thought out level designs to detailed backgrounds and some excellent dialog and voice work.
I'm waxing lyrical a bit about Iji, because I do like it a lot, but I'm about to lay into one of it's biggest shortfalls. Iji offers two different play styles. Whilst the choice between going in guns blazing and sneak pacifism isn't exactly new, Iji really shines when it comes to showing it's effects on the world and the character in lots of subtle detail. So much of the dialog and set pieces can change depending on how many enemies you killed. In particular one of my favorite touches is that Iji's voice 'barks' change. If you kill an enemy when you have been avoiding kills Iji will let out a forlorn and desperate "Sorry". Dispatching and tough enemy when you have been killing everything in your path and Iji scream "Die" in a desperate berserk fashion that eventually made me feel like she might just start pouncing on enemies and savagely beating them in the face without any input from me.
The problem is that the pacifism doesn't really mesh correctly with Iji's skills, even though some of them seem like they would be useful they turn out to be less used than when you are playing as a strait shooter. I'll just do a quick rundown
Jump - Moving around, avoiding enemy fire.
Duck - Avoiding enemy fire.
Kick - Get through shield doors. Knock back enemies, but at the risk of accidentally killing them
Hack - Get through security doors. Hack enemies to disable weapons but at risk of getting a kill.
Weapons - Certain ones have specific uses for reaching areas on the level and bosses.
Of those skills only one is fairly useless when playing in an aggressive way (Hack, as you need be behind an un-alert enemy to do it) where as nearly all of them have a fairly specific use in pacifism aside from jumping. It makes pacifism so much of a shallow gameplay experience because instead of the intense firefights mostly you jump past enemies and kick/hack down the occasional door you wouldn't have in an aggressive mode (although Kick gets just as much use in aggressive for enemies).
This is highlighted in a level where if you are playing pacifist the enemies will ally themselves with you as long as you don't touch/shoot them or break nearby doors. If there was only some kind of ability you could invest points in that would help you avoid enemies, like rolling, dodging, ceiling hanging or something, it wouldn't be the frustrating experience of waiting for an enemy to move and then jumping over his head. It was interesting being offered a different experience from the same game, but that experience wasn't nearly as fun as shooting everything that moves because there is lot less variety.
I think I find it extra frustrating as so much thought has been put into making the skills useful for both gameplay modes. Only one of the seven stats you can improve is almost exclusively useful to an aggressive play style. Everything else not only multiple use, but all the uses fit within the game logic. It's worth noting that pacifism is still fun, markedly more than in a lot of other games that offer it as a play option. It's worth doing all the jumping and ducking just to see all the differences it makes to the story.
The moral of this story is if you offer more than one emergent play style it's important to make sure that the players abilities support it throughout and not just in particular circumstance. Otherwise you risk the choice becoming a trade off between rewards and fun, rather than one of pure personal preference.