It's hugely disappointing. Final Cut Pro, Motion, Compressor, and Logic Pro all got significant updates yesterday. I wouldn't consider the Aperture 3.5 update of a few months back in the same ballpark, and Aperture's starting from a much older code base. Final Cut Pro and Logic, I think, are Apple's statement what a video and a music editing program should be. I can't say the same for Aperture now - it's just gone a bit stale.
I can think of a few reasons for the delay in Aperture 4/X, none of them good:
1. It's not ready. Probably the most likely explanation. But if not now, when? The team surely knew about the December deadline for months. The other "pro" app teams all managed to hit it. Aperture 3.x is almost four years old; the last major-ish update - 3.3 - was more than a year ago. If 4/X wasn't good to go now, what's the earliest possibility?
2. They're waiting on some iPad companion app. Possible, but then who knows when everything will be ready? This is potentially the best explanation as the delay could just be an announcement timing issue: Mac Pro and the other "pro" apps now, Aperture and companion sometime in the new year.
3. Apple thinks Aperture 3.x is fine. It's a very good program, but I sure hope they don't think it's complete. The Aperture team are surely not dummies, and they're well aware of what Lightroom - and Final Cut! - can do, so I don't find this likely, but it would be hugely disappointing if true.