The wife and I played both Skylanders games, an have purchased tons of figures. Not being able to play whichever Disney figures you own in whatever adventure you want totally kills this for us. User generated stuff has zero appeal.
That is a let down but it is part of the design.
You know Skylanders have all the stuff on the disc itself, and the figures just unlock the characters. They have all planned from the start.
With Infinity the devs would have to worry about different characters in playsets they do not belong, not only at the design of the base game, but later when new playsets are introduced. I imagined that the six initial playsets are just like Skylanders, already on disc, but later more can be introduced as updates. So a new playset character would impact older playsets if the characters are allowed to cross the limits of the playsets. Huge tasks, huge DLCs.
The use of any character in any playset could be great but also could limit what you do in that particular playset. For that the sandbox portion exists. The sandbox can be updated everytime a new playset is released.
I also have zero interest into building stuff (do not have time for time) but I am sure there will be great creations, not only from the players, but also from the devs, to cover the lack of multi-characters playsets.