One other thing about incompatibility, and this is why I'm not bumping my 10 speed bikes up to 11 speed any time soon, is that it would make my 3 or 4 wheelsets obsolete because you can't put an 11 speed cassette on a Shimano Ultegra/105 rear hub that was capable of a 9 or 10 speed cluster, in fact you can't even just replace the original freewheel portion of the hub with an 11 speed compatible freewheel. and make it work.
Again, thank you Shimano

Edit: I just was told by a friend that there is a way you can make your 9/10 speed wheels work with an 11 speed cassette and that is by purchasing a small independent company's proprietary 11 speed cassette called an Edco Monoblock. It is machined out of a single piece of steel and will supposedly work on a Shimano or Campy hub (they have versions for each). Of course at $225 for a single cassette you might just consider rebuilding your old wheel with an 11 speed hub but I thought I would post this for completeness sake.