Before i start, i have to admit i am no expert on the Batman comics, ive only ever read one Annual and Bruce spent most of that pretending to be Matches Malone whilst Robin ran around in an inflatable muscle suit (!?!). So my perception of his rogues gallery is very much based on various wikipedia summaries and the previous filmed works.
But with that caveat i think the main problem with Batman Forever is that its creative team made the incorrect assumption that if something is based on a comic, it cannot be played seriously.
Which is why they took a villain like Two-Face, and turned him into The Joker Mk.II. The Dark Knight has proven that Two-Face can be an effective and intimidating villain. Instead we got a rehash of The Joker sans any of the darkness and menace.
The Riddler on the other hand didnt even appear in Batman Forever. Instead we got the Cable Guy in lycra. Carrey can be a very good dramatic actor, but all we got in Forever was a mix of Ace Ventura and The Mask in an outfit that left very little to the imagination.
We ended up with a Robin that goes on the pull in the Batmobile a few days after watching his entire family die.
We got a Batman that spends his spare time stalking a woman with a rubber fetish. And then gives up being Batman altogether when said female decides she doesnt like rubber as much as she thought, and would rather date a dull guy in a turtleneck.
As a kid i loved Batman Forever, as an adult i find it almost unwatchable. About the only positive i can say is that its not as bad as Batman and Robin.