A lot of games these days are going the way of being open world 'sandbox' style. The first that comes to mind is the GTA series and maybe others like Oblivion and Fallout, which I haven't played.
Maybe it's just me but I really can't seem to get into this type of game. I've had GTAIV installed on Steam for months and just barely scratched the surface of the content. It's probably because I spend most of my time running over pedestrians. But that is exactly the problem with open world games: too much freedom.
I don't like side quests or talking to random NPCs whose sole purpose in life is to stand around waiting on you to solve their problems. And the thing is, you may be able to kill everyone else except the annoying NPCs.
Linear games are often shot down for being, well, too linear. Personally I like being guided along a game's story line. Pretty much every FPS has you run>shoot>cutscene rinse and repeat. Is that such a bad thing? To me it's like an interactive movie or a fine work of art. A well written game will be awesome even if it is linear.
I don't get the craze behind Half Life 2, but it's linear. CoD is linear. Just give me a gun and point me in the right direction.