The third game didn't hold a ton of appeal for me. I have it, I got it as part of one of the Humble Bundles at one point, but I have yet to play it. The zany antics took a front seat. You even have a melee weapon shaped like a giant dildo. The dick and...

I think the big thing with the Saint's Row games has been a progressively more outrageous attitude of "Why? Because you can!" It's a fantasy version of GTA, which I've always appreciated, even though I've never played more than a few hours of any Saint's game. This 4th iteration is a little more appealing to me, mostly because of the super powers. Well...and the dubstep gun. That's just genius.
I did play an hour or so of SR3 after posting this, and I have to admit it's quite a blast so far. I mean I had a gunfight while in freefall. When does that happen?
"On the one hand, this seems like a game trying to be too much, throwing in so many different mechanics that it becomes a muddled mess."
That's pretty close to what I said about SR3 before it came out. I anticipated that there would just be too many elements thrown together haphazardly. Instead, I discovered the game had too many elements very carefully thrown together. SR3 is over-the-top without going too far into the realm of absurdity to make it a caricature of its own cleverness.
Still, not as much fun after the missions are complete as SR2. SR2 remains playable LONG after the story missions are done.