Same here, I left the theater thinking "What the hell was that?". It definitely didn't live up to the second film.
The Punisher and Wolverine's movie rights are owned by Fox, so you'll never see them in a Spidey movie. The Punisher would have been cool, but Wolverine sucks. Has he ever won a fight in the 3 X-Men movies?
No more Goblins either! Having them appear in 3 out of 4 films would be insane.
The Lizard makes a great villain because Spider-Man knows he can't just beat the hell out of him because he'd be hurting Doc Conners. He always has to hold back, and that could get him killed.
The Sinister Six? Weren't they just six lame guys that couldn't beat Spider-Man individually?
Electro could be alright, as long as they didn't use his costume. I just picture a guy walking around with blue static electricity flowing all around him.
Scorpion is probably my favorite. A former cop selected by J. Jonah Jameson to be a hero and overshadow Spider-Man, instead goes crazy and becomes a villain.
Personally, the crossover I'd love to see is Spider-Man vs Daredevil. Until they realize they're both on the same side and go after the Kingpin together. But again, Fox has the rights to Daredevil and the Kingpin.