Being a long time Spider-Man fan (I started reading Spider-Man comics back in 1977), I would have to say I like the movie very much (though I have only seen it three times so far).
On the subject of the Green Goblin, moving from Peter's beginnings to Norman's death took 112 issues of Amazing Spider-Man, and I think they did a good job of working it all together. I remember reading early scripts of Batman (when they still thought that they were going to include Robin in the first movie) thinking the same thing. The next Goblin needs to be Harry, and then (at some later date) they can bring back Norman.
Most of the people I've talked to think that Dr. Octavius is going to be in the next movie, but we shouldn't rule out Dr. Connors (who fired Peter in this movie from his lab job) either. Also Peter dates quite a few girls (including Betty Brant) before marrying Mary Jane. The number of twists and turns that could be played out should keep us guessing for quite some time.
The one thing we don't need to guess about is that there is going to be more. While waiting to see it for the third time, the manager of the theatre came in and told us that Spider-Man had reach $200 million... three days earlier than the Phantom Menace did.