Personally, I thought that the environment and freedom of movement were, especially for the time in which this game came out, among the best parts of the game, and the collectibles served to show off those aspects. I didn't mind them.
You know what game had tedious, pointless achievements? And I'm talking about a quality game, not some crap that everyone expected to be a pain to play.
Mafia 2. Go drive a thousand miles in that game, and then after a minimum of 10 hours without saving (and that's a hard minimum; if you get it in ten hours then you have the concentration of a Jedi and the luck of a Leprechaun), see if you still hate Assassin's Creed. At least Altair doesn't need gas every half hour, and can't lose a hundred flags' worth of progress if a single guard happens to attack him.