After playing through Saint's Row I expect GTA's gameplay to live up to it. SR itself wasn't a fantastic game, but it brought a lot of good ideas to the "open sandbox gangster" style videogames. For one, the map could be marked with way points which made driving around a lot easier than "PRESS SELECT CHECK MAP DRIVE A LITTLE BIT PRESS SELECT CHECK MAP" and the game's dual analog control set up actually made combat FUN.
The map marker/GPS system has been confirmed and by description seems identical to the SR one. Also GTA4 has free aiming now (which isn't as broken as SA's), both in person and in car and I think I read that they've learned from SR as well that players should be able to use other weapons than just small automatic weapon for drive-by's (I think I read, again, somewhere that all weapons are available from the car but only the one-handed can be used while driving (does this mean increased AI driving? That you can GPS a location and you can get a gang member to drive you???))
SR did a lot of small things that made it superior to GTA imo. Free aiming is the most important one, but stuff like way better sidemissions (not all were good, but stuff like Insurance Fraud was way ahead any sidemissions I played in the GTA series), having a lot of information on screen (what gangs are currently gunning for you, how the police meter is building up, seeing the run bar increase and decrease as you run/stop), not really having any load times in-game (which opened up a shitload of gameplay for me that was in SA as well but that I never ever touched in there because it took forever, like car tuning and going into shops for buying stuff) and character customization (which was pulled way better off in Saints Row than in SA, but tbh this doesn't matter that much as long as the characters are as fleshed out as Niko seems to be).
That said, I do prefer the GTA series (especially Vice City, SA was a bit of a disappointment as it seemed a bit blander). So now that it seems GTA4 is going to take the good parts from previous GTA installments (character development, style) and mix it with elements from SR (free aiming and GPS have both been confirmed) and add a little Gears of War-style cover mechanics plus multiplayer and I think we have a winner. Considering that SA was a bit of a disappointment, it was still a great game and I doubt GTA4 will be anything but it.
And ya I already pre-ordered mine plus a strategy book 8) (too lazy to find all the damned hidden packages/tags etc and I'm not gonna wait for some dude to gamefag it)
edit: oh yeah Saints Row also has way better visuals and audios on quite a few levels as well. Gunfire in Saints Row sounds very satisfying and I feel like my guns pack quite the punch. And the explosions are probably as awesome as any game I've played.