I'll admit I'm a bit disappointed, but at the same time just getting to see Tennant's Doc again sounds pretty damn cool.
And I don't buy that it's because Moffat has no respect for the past. He probably just thought it would be a lot more fun to watch these two Doctors go on an adventure together, than to have another special with 4 or 5 Doctors talking and bickering endlessly for an hour (which is usually what tends to happen in these things).
And as others have said, there's many more ways to pay respect to the history of the show than by just trotting out the original actors. In fact that's probably the least original thing you could do.
Remember, the notion that you'll have 4-5 Doctors if you open it up to classic Doctors is just a red herring. You can include just one in a major capacity. And, the bickering is only present if Moffat writes it that way. Obviously, he can write it to not include the bickering.
Including the original actors is the most honest way to celebrate their contributions.
But, I agree that Moffat does respect the classic series. That just makes it all the more puzzling to me.