Orlando is themed restaurant city and there are plenty of good ones, but Hard Rock Cafe is the original and, for me, still the best.
Race Rock on I-Drive is worth a visit, but we've done it to death now. We like Rainforest Cafe (we usually eat at the one at AK as the other food offerings there are dire, IMO), but it receives mixed reviews - you either love it or hate it. It tends to work out a bit more expensive than the others, but the servings are huge and you could easily share an entree.
Nascar cafe and NBA City at CityWalk are just OK - they lack atmosphere for me. My teenage girls loved Motown Cafe (also at CW), but I've never tried it.
Dan Marino's Town Tavern at Pointe Orlando is quite good, Buca di Beppo at Florida Mall is a themed restaurant and a half (really over the top kitsch), 50s Prime Time at MGM was good fun, but I'm not so keen on the Sci-Fi Dine-In. I agree with Mike about the B-Line Diner, but I'm not sure that I'd necessarily put it in the themed restaurant genre.
Bahama Breeze gets rave reviews (not sure if this is considered a theme restaurant either), but I hated it. Great atmosphere, but far too crowded and mediocre food (and that's being generous).
I could go on and on!