Barals Expertise and Crush of Tentacles are quite expensive, so your deck already needs to be built to survive and go long, which both of these cards do more of
Nulldrifter is obviously a take on Mulldrifter, and tacking annihilator onto it is a way to hammer home the Eldrazi nature of it
As with the Delver, the catcher will get stronger if other tempo and aggressive blue creatures are printed
While the Japanese version is arguably the most popular version, the English version is just as satisfying and no less dark (I'd post a hyperlink there, but that version of the scene seems to have been removed from YouTube)
Examine the seal inside the foil packaging
The Ur-Dragon also has a game-ending effect once it hits the board, refilling your hand and slamming more dragons into play for free