Stealth has always been a favorite of mine when it comes to game mechanics. It's a nice departure from the usual "point guns on enemies untill they fall over" mindset that most games tend to lean towards. Stealth also seems like the more sensible approach, and most real life combat situations are heavily focused on getting what you want without being seen.
Sneaking past a variety of henchmen loaded to the teeth with the latest weapons and armor, gives you a great sense of arrogance and downright douchebaggery (It's a word now, deal with it!), but it also makes you feel vastly more powerful than an AK-47 ever will. The reason for this is helped a lot by games like Splinter Cell: Conviction and Batman (the good one), where enemies will practically pee their pants in anticipation as their buddies disappear one by one. Still, my favorite form of stealth is when going completely unnoticed, and when they know I'm there it takes some of the fun out of it, and it feels somewhat like a failure.
The reason for writing this was partly because of an article I read on GamesRadar called "Things I'm sick of games making me do". Among these were tedious chores like hacking and lockpicking, but also stealth in games that werent centered around it. Now, I like Metal Gear Solid and Splinter Cell as much as the next guy, but I seem to enjoy games where stealth aren't a core part of the experience to be as, and in some cases more enjoyable. Of course there are exceptions like Uncharted 2, but that is because it doesn't work well to begin with, and the enemy AI cocks it up anyway. On the other hand games like, Fallout 3 and Oblivion, I really like because it doesn't force you to be stealthy, and the environment isn't designed to be "stealth-friendly" like... say... Splinter Cell: Conviction.
What I love the most (probably) about being sneaky in for example Fallout 3, is that whenever I sneak past a super mutant or plant a grenade in someones pocket, I always feel like I'm cheating the system in some way. It's kind of difficult to put in writing, but it's really satisfying to murder an entire town without anyone ever knowing. It's like the perfect crime.