Since they seem to have completed the whole opening level, that means most of the story mechanics are worked out.

 They mention customization coming into play on the second time through, so they have most of the later levels layed out and mostly worked out, might be tuning needed on a few creature spawns and such but still, mostly done.

 They have most of their co-op set up worked out, even with them considering some of the additional demands players are adding in.

 Their multiplayer is probably where some of the hold up is.  Finishing the maps, adjusting weapon spawns, checking for easy glitches and for mis-sized paths.  Rounding out their connection issues, their matchmaking board, the "points system", partying, and all the requests from fans.

 Who knows, they might be able to release a final product next week.  But more likely they are going to work on it at least two more months.  That wisp of a date of February 23 is probably the lastest date possible, short of a massive problem cropping up.  But I wouldn't expect it more then a month earlier then that, since we do want a fully rounded game when it comes.