There are very few movie sequels that I have loved. I loved Matrix. I didn't love the sequels. In fact, I thought the third and last one was a complete waste of time with all it's messianic-references.
The same goes for Alien and Terminator - loved the originality of first one, hated the straight-forwardness of the sequels
Sometimes it's better just to let a story finish without trying to explain the 'verse it inhabits (I'm talking 'bout you, Matrix).
With Alien, the monster surprise was no longer a surprise.
With Terminator, maybe the novelty just wore off.
And yet..and yet...I loved all the Die Hard movies. Why is it that I did not get tired of watching Bruce Willis getting older and balder and getting his body beaten up all the time? He's not pretty like Johnny Depp. Maybe when a sequel that is really just another story with the same main characters as the previous film, it works.
Harry Potter - yes I have enjoyed every single one, despite whinging about how Hermione's really not supposed to be that pretty, and all those stereotypes about fat boys, and all that boring quidditch. These stories were all designed to follow on from each other, conceptualised from the start by the author. I guess that's why these work whilc others don't.
Lord of the Rings. It's really one story split up into 3 movies, which is similar to Harry Potter.
Anyone else got any theories?