In MGS2, Raiden was whiney and annoying. But the real reason so many hated him was because he wasn't Snake. Going into MGS2, you're fooled into thinking you'll follow Snake around the whole time, only to have him replaced by a far less likable character less than 2 hours in. It was a big "screw you" to a lot of MGS1 fans who wanted to play as their favorite character again. After all, the whole series is really about Snake, not some kid that apparently can't decorate his room (Really, did we need to sit through 5 minutes of Rose crying about Raiden's undecorated room?).
In MGS4, on the other hand, Raiden is relegated to a being a side character (which he should have been in the first place), so he can shine with some cool, over-the-top, moments.