Having now read the Dungeon Crawl Classics beta document, and observed the whirlstorm and piñata party of posting around it, I find that most things I want to say about it have already been said.
My personal reaction is: it could be great fun as a convention one-shot, but not something I'd use or even plunder for a campaign. The game is indeed old school, but think 1980 rather than 1975, and a 1980 that never existed at that. Imagine if every producer of heartbreakers and complications for the ol' roleplaying chassis had asked, not "How do I make this more realistic?" but "How do I make this more crazy and fun?" Imagine if in 2008, the Wizards team had asked, not "How do I make this more balanced and tactical?" but "How do I make this more crazy and fun?" That is certainly the obvious aim of this game. I'll reserve judgment on whether it is actual fun until such time as I sit down and play.
Also, I especially enjoyed the line drawings by Peter Mullen. They are like Sidney Sime's material as drawn by Tove Jansson, in other words, double weird fantastic.
But I want to focus in on one specific reaction I had (and Mr. Rients, too, in his scrawlings). I really liked the idea that fighters had a special power that they could use with some of their attack rolls, to do improvised maneuvers like knock an enemy back. The "Mighty Feats of Arms" in the player's section are great, and presented in the right amount of detail.
Then you get to the judge's section. And each MFoA is presented in excruciating detail, with a paragraph detailing the exact result in game terms for each level of success rolled.
Once you have written a rule you cannot unwrite it. It doesn't matter if you have encouraged the judge and players to "be creative, hey presto old school." The space for that creativity has to be kept open by not making any kind of official rule. Otherwise, whatever you improvise might be seen, on a later perusal, to be "wrong" or lacking.
In other words, if this was my reaction to the Mighty Feats rule for the players:
By the way, if you want to implement "Mighty feats" in a more standard D&D-style game without rolling an extra die, a quick solution might be to roll the fighter's weapon damage die whether or not the hit succeeds, and have a feat-style effect happen on a natural 1 or 2. This advantages the dagger and other low-damage weapons, but also gives somewhat of a consolation prize for missing in combat.