What's the difference?
The short answer is that "it's hard to explain," but that's not really an answer. First of all, in many ways, it's a park just like Disneyland in the sense that it has several themed "lands" that radiate from a central point. Except in this park, that central point is a giant volcano called Mount Prometheus, which itself holds another land, the Jules Verne-inspired Mysterious Island. The other lands include the American Waterfront, the European-style Mediterranean Harbor, Port Discovery, the Lost River Delta, the Arabian Coast, and the Mermaid Lagoon. Each of these lands have themed restaurants, shops, and rides, many of which incorporate Disney characters from the "classic" line-up or the movies.
The main difference is that the park is themed on a level far and above that of any Disney park I've been to (and I'm only missing one continent!). Each land is so immersive that you'd be hard-pressed to remember you're in a fun park. The other difference is that DisneySea has a reputation for skewing to an "older" crowd, as opposed to the "kiddie" fare of Disneyland. Now, this really means you'll just find more teenagers on first dates rather than families with small children, but it does takes some level of maturity to really soak in the Atmosphere more than the Stuff To Do.
What's this about Stuff To Do?
As much as I could praise DisneySea for theming, presence, and atmosphere, where it really doesn't hold up is the Stuff To Do. Your chances for jet coasters are even more limited than Disneyland, as there are only two truly E-Ticket class attractions for the entire park-- the Tower of Terror and the Indiana Jones Adventure. There are other rides, yes, like the (rather dull) Raging Spirits coaster and the (more innovative) Journey to the Center of the Earth, but these rides feel more like missed opportunities than completely satisfying experiences. It's especially noticeable in an area like the Arabian Coast, which has one 3-D show, one Small World-like dark ride, one carousel, one restaurant and a few merchandise stalls-- that's pretty much about it, all in the square footage of the entire Fantasyland, which contains twice that.
There are more reliances on live entertainment than in Disneyland, with several shows happening in dedicated venues and two elaborate "lagoon shows--" one during the day and one at night.
So is it worth it?
It's definitely an experience worth having. The plus side is that DisneySea is nowhere near as crowded at Disneyland while taking up the same amount of space, so it feels incredibly different. (Except during the lagoon shows, in which everyone crowds around the lagoon area.) The popular rides (like Tower of Terror and Indiana Jones) also have single rider lines in addition to Fastpasses, giving you another time-saving option. Heck, the biggest line you'd probably get in is for the gyoza bun at the Mysterious Island (which, incidentally, is totally worth it.)
Because of the lack of crowds (and lack of attractions) my strongest suggestion is to take advantage of the Starlight Passport on a weekend/holiday, getting in after 3 pm for a reduced rate. The same thing can only be done after 6 weekdays (and it's even cheaper) which still gives you about 4 hours to complete the park. The good news is that you could pretty much do every ride you wanted in that time, but the bad news is that you miss out on some show opportunities, including the daytime lagoon show, The Legend of Mythica.
One of the neat things about the park is that it's so familiar (being Disney, after all) but so unique at the same time. For example, "our" Tower of Terror is different than everyone else's, because it features the shenanigans of a li'l demon called Shirikir Utundu. And there's a Sinbad ride that looks like a great Disney film never made. But it's more than the buildings and sets. Even the attendants all get in on the act, really embracing what it means to be an actual part of each attraction. It's what we all hoped themed parks could be. So all in all, you can't help but feel that you are a part of something special, something that no one else has, and it's on a scale that no one can ever repeat again, (not in this economic lifetime anyway.)
So who wants to go? :)