I think you guys are being a little hard on the game, especially after only playing a few hours. I've played 6 hours so far, and although it does feel quite similar to the first, the story itself has started to come into its own and with the new tonics and powers you unlock as you progress, it is definitely worth sticking with. It also retains the charm and atmosphere of the first; perhaps not as revolutionary in terms of changes, but I certainly wouldn't say that it was boring or more of the same, it is a worthy sequel.
I don't see a problem with it adopting the same engine/models; the first game was that good that it was always going to be tough to bring out a sequel, and I would rather it build upon the successes of the first and expand the story than try too hard to be different and completely change the game mechanics.
It probably could have been released as DLC, especially in light of the content delivered with GTA, but I suppose the multiplayer elements and various optional missions built into the game made it worth a full release.