One nice thing about this game is that it doesn't get tiring or overly repetetive. The scenery changes and they intersperse the various types of play (Jeep, jet ski, puzzles, etc...) IMO, it is well scripted to keep you interested; something I can't say about all games - the story progresses at a good pace and does not seem to have moments of lull or ridiculously difficult "mini-bosses" that bring the game to a near stand still. Naughty Dog really knows how to make a game entertaining.
The environments are not as physically detailed and free roaming as Assassin's Creed, but they are far more advanced in their appearance and have a life all their own as mentioned by Chris.
I also find it really funny that they gave Drake such a real personality...there is a section in the very first area where he has to walk across a dilapidated bridge. I was thinking to myself "THAT looks safe...
" and as you walk up to it, Drake says out loud that exact statement LOL. Small things like that go a long way to add to the feel of the game.