Most of my points have been covered, but I’ll add some color commentary:
Not many “timed” missions per se, but a lot of chase missions, which I hate. I hate them for two reasons: A) I suck at driving GTA, and B) it introduces the same factor that timed missions do, in that tactics go out the window and it is all about twitch.
As to save-on-demand during missions: I understand their design decision, but reject it. The choices aren’t “save whenever you want” vs. the GTA no-save-until-the-end; a middle ground of “checkpoints” is sound (such as that employed by Call of Duty, among others). There have been several times where I literally threw down the controller in disgust, and just shut everything off for the night, because I was 23 minutes into a 25-minute mission when I: fell off a building/missed a turn/Niko throws a grenade at his feet.
Especially this:
At the end, you have the big shootout in the old building, then have to (depending on your choice of revenge/money) either race the motorcycle and hit the jump squarely and at the right speed, or maneuver your boat under a helicopter, was extremely frustrating. I did the boat/copter one first, and when trying to maneuver under the too-low chopper I ended up bumping the chopper about a mile away. I was pretty pissed.
For offline sandbox play, I prefer Saint’s Row…it is just more fun for me. The available cheats are more fun (homicidal cars, for instance) for sandboxing. In GTA IV, once you get to 3+ stars the cops are brutal; in Saint’s Row they’re more manageable, in that they come in pairs rather than swarms.