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I loove stealth games. I've played just about every franchise that has stealth (with exception to MGS), and I love them so much. Maybe its just a subconscious desire to be that hidden assassin, stealthily taking lives. Stealth games are up there along with horror games in that the quality of the visuals and sounds must be superb. Players are naturally tense, and are watching for the slightest giveaway to indicate them being spotted or an enemy in the distance.
But I think the most important factor in a stealth (and horror) game is the level design. Each level is nothing more than a giant, moving puzzle filled with cameras, lights, shadows, patrolling guards, stationary guards, and you have to navigate it all with almost total perfection. Bad level design on a game focusing on stealth is crippling, since that's the entire focus on the game.
I consider the best stealth game of all time to be Thief 2. The main attraction of the game is that each level is quite open ended and sprawling. There are usually several ways to go about your main objective, let alone all the swag you are rewarded for finding if you poke through every nook and cranny. Levels are like labyrinthe's (literally, too, in one instance), and killing people is extremely discouraged.
Splinter Cell is arguably on the opposite end of the spectrum. Levels are very linear, and you're free to kill with wanton, provided you have enough bullets and don't set off any alarms. While this isn't necessarily bad, as the levels are done pretty well, but Thief just has that empowering feeling of openness.
I'm unsure how Conviction will conduct itself (nor do I care much about MGS, but I hear its good), but I haven't seen anything in the way of an open world environment. I encourage you to play Thief 2 (not 3) and experience it for yourself.