Saints Row just lets you get away with having more fun than GTA IV did. In Saints Row, you can run all over town chucking grenades all over the place, and while the authorities will show up, they're not particularly effectual and are more likely to become victims of your rampage than to stop you. In GTA IV it's like, cops get on your ass if you break simple traffic laws, and if you don't focus 100% on losing them your ass is going down.
Okay, maybe it's not quite THAT bad, but you can't argue that GTA IV punishes you more severely and for smaller offenses than Saints Row does.
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Black Turtle spilled my diet soda!