Every day they don't announce it the bad rumors pile up. At their reveal I feel like all they'll be doing is quashing bad rumors, and that's best case if none of them are true! Kinect 2 won't do it because people will just say they already have Kinect. Specs won't do it because it's not going to look noticeably better than what PS4 has already shown us. The only chance they have to have a real "wow" reveal is reveal a bunch of exclusives, but with Halo 4 just last year and Gears out just now, they will be new IPs and from new studios or Kinect games so will the hardcore have interest?
I'm not saying they won't look good or be good but that leaves little track record and little room for a WOW announcement. Also they have to save some games for E3. Streaming, sharing, spectating, graphics demos, big specs, these have all been shown for a console coming this holiday so how does MS show these features without looking like a "me too" announcement.
I'm not sure what sort of surprise they can get us with, but I guess it wouldn't be a surprise if I knew what it was. Guess we'll have to hope that "secret sauce" is really special. Also BF and COD won't do it because they're on both unless MS really feels like breaking the entertainment division budget lol. They'll probably make a big deal about showing the form factor (like rubbing it in Sony's face as if it actually makes the console any better) and act like 8gb of DDR3 is the same as GDDR5 because they won't mention what type.