Nipples on the Batsuit. Robin being a jackass. Bruce Wayne giving his identity away like there's no big deal to it. Nipples on the Batsuit. The complex character of Two-Face being made completely one dimensional. Giving the Riddler an alter-ego. He's a villain, for crying out loud. Nipples on the Batsuit.
See the pattern?
While more "fun" (i.e. less dark) than Batman Returns, Batman Forever had a new caped crusader (Val Kilmer) who seemed ill at ease, and a couple of villains that were at best, in the case of Jim Carrey's Riddler, just second-stringers. Whereas Nicholson's Joker was homicidal, Tommy Lee Jones' Two-Face was just a brute and the Riddler nothing more than a disgruntled employee with a thing for television. It was lame.
Garishly colored (although the sets were still pretty cool) to look like a comic book - but not a Batman comic, mind you; it's Gotham City, remember? - and absurdly written, Batman Forever was a major disappointment.It just can't compare to the first one.