I hypothesized that he was seriously off his game because he was cut off from the TARDIS. But that's just me.

Apparently there is no real reason for there to be a "Doctor-lite" episode. I've heard - and this makes sense - that it had something to do with Tennant's contract, wherein he was contracted for thirteen episodes but that didn't include the Christmas special, so they dealt with it by giving him one episode of almost no work. I don't think it'd make any sense at all for him not to appear in one half of the finale, and I don't think that's what's going to happen.

Though personally speaking, I think a River-and-Pond roadshow would be awesome.