These are things GTAV NEEDS to steal from Saints Row series that makes it ever so hard to go back to GTA.
1. UNIVERSAL GARAGES. Any car you put in your garage is saved. FOREVER. And you can access your list of cars from any garage in the game. If a car blows up, guess what? You pay a small fee to have it revived. If you take it out for a spin and don't take it back, guess what? It just reloads in your garage later. On my GTAIV replay I have used the cars I have on my parking spot maybe a total of 5 times. Keeping cars in GTA is stupid because even if you customize it (of course, GTAIV doesn't let you customize cars even though you could in San Andreas!) if it's destroyed it's lost...FOREVER. Not in Saints Row! Especially with its car customization.
2. Allies. This was in GTASA, but Saints Row expands upon it. Any time you see a gang member of the Saints, you just press up and they automatically join you. But what's more, is that you can call in story related characters and even unlock hidden homies to tag along in case you feel a little lonely while you wreck havok.
3. MISSION CHECKPOINTS. I die in GTAIV, I have to start at the beginning of the mission. I die in Saints Row 2, I get to start from the latest checkpoint and missions usually have 2-3 checkpoints a piece. Furthermore, once you've started a mission you can cancel at any time. When you die or get arrested in mid mission you don't land in the hospital or the police station, you get a screen that tells you you failed the mission and gives the option to quit the mission, restart from the beginning, or restart from checkpoint. Very convenient.
4. Weapon cache. As your weapon arsenal grows, you don't need to go to the store anymore for weapons, because they're stocked at your safehouse. All you have to do is go to your house, go to the weapon cache and click whichever weapon you prefer.
5. Cruise control. In a car chase that needs a little gunfire? Hit cruise control to make aiming better.
6. Can save anywhere, so long as not in a mission.
7. Co-op.
And while we're on the subject, things GTAV could learn from Red Dead Redemption:
1. Random events. A gang of robbers rob a town. A man is beating a woman in an alley. Some guys are chasing after another dude and you want to know WHY. A coyote just mauled your horse, ate it, and left you in the middle of the desert with nothing but a six shooter.
2. Better stranger missions that have multiple chapters.
3. Random missions found by exploring.
4. Fast travel, ability to save anywhere.
I can't think of anything else because I'm tired.