Can you elaborate on what you're referring to? Blizzard has delayed 'major' features in every single game (and expansion) they've released since D2 (and likely every other game before that, including lost vikings ; ).
That's just it - they are not leaving this as a "when it's done"-type thing; from the sound of it, they are releasing the game in sort of a "piecemeal" fashion to at least get the "finished" part out the door*. In the past they shipped a full, feature-complete product at a time of their choosing. Who knows, this may be an improvement - Blizzard games always have taken an aggravatingly long time to come out. This might be a good choice for them.
* I suppose one could make a case that splitting the Starcraft 2 campaign into 3 parts was another instance of this, so maybe it isn't exactly the first time.
“The greatness of a nation and its moral progress can be judged by the way its animals are treated.” - Mahatma Gandhi