I explained that. The Riddler is not a ranting maniac like the Joker. He's sane, cunning, and calculating, aside from his obsession (in some versions) with constantly leaving clues for Batman. I've seen it said that in some ways he's the ideal villain for a master detective like Batman, because he's all about creating mysteries and puzzles to challenge Batman's deductive skills. So Nygma should operate on a more erudite, intellectual plane. He's in it for the challenge, matching wits with the World's Greatest Detective and trying to stump him.
There's also a debonair quality to the best Riddlers, like John Glover's B:TAS character or Frank Gorshin in the suit-and-bowler-hat outfit that the actor preferred over the original green tights. After all, the character's design in the comics was (I think) modeled on Fred Astaire, so there's a certain elegance built into him.
Granted, Gorshin's Riddler was prone to cackling maniacally and putting on a good rant; indeed, he was an influence on Mark Hamill's Joker. He got a kick out of his villainy, as did most of that show's villains, but there was always a sense of calculating menace and ruthlessness underneath.
Carrey would've made an excellent Joker -- certainly a damn sight better than Jack Nicholson, who didn't even try to play the Joker but just played Jack Nicholson in whiteface. But the movie was wrong to reduce the Riddler to just a Joker knockoff. That's not what he is.
