This is an interesting conversation. I definitely like the idea of sequels and trilogies. It gives you the chance to hook in readers, but not for too long, as someone earlier in this post mentioned, as by the end they might grow out of them before you're finished writing. I read UGLIES and PRETTIES as they came out, but by the time SPECIALS came out I was too old, and I just saw EXTRAS at Barnes and Noble the other day. I liked GOSSIP GIRL when it was new and clever, but when publishers got the idea that product placement in books is about a million times more effective than television commercials, they started to get bad. And I stopped reading the series long before it was over.
I think capping a good story with good characters at about three books keeps you respected, too. I lost a lot of respect for writers who "sold out" and kept writing--sure, money's nice, but if you're clever enough to come up with one good story, can't we count on you to know when to stop it and to be clever enough to come up with a new one? That's why writing about the same world but not doing a direct sequel is a good idea, I think. I noticed Gail Carson Levine's book FAIREST takes place in Ayortha (I think? I haven't read it yet), the country south of Ella's in ELLA ENCHANTED. I think that's a great idea, because you can always grab a new reader, because it's a standalone book, but fans who wished for "sequels" (I think usually when we want sequels, we don't actually--we just want the experience of reading the book again for the first time) can also enjoy it a lot.