the way LBP is now, the experience is pigeonholed into side-scrolling gameplay. this will have some serious impact on long term motivation. there are only so many things you can do with side-scrolling gameplay and the industry started with side-scrolling games 30 years ago!
Maybe, but don't forget that LBP has a lot of features not supported by most other side-scrolling games ... (It also doesn't support everything every side-scrolling game has ever supported, mind you). Also, very few side-scrollers supported up to four players, on or offline, realtime physics, etc. ...
And very important, the combination of the "rapid development tools" and the probably tens of thousands of people who will be building stuff for this game, the ease of working together whereever you are in the world, the openness of the community, and the expansibility (MM will be adding a lot of stuff over the next year I'm sure), makes this really something that's more than just a 'build your own super mario bros 1 clone'.
i think LBP will have a very strong but short pike of success. you just don't have enough options to support new ideas and gameplay over the course of months or even years!
I don't think you've given the available options or even available online levels nearly enough time to make this judgment!