I started as a zone-press team and loved the flexibility (you can run 2-3, 3-2, 2-3 w. press, 3-2 w. press or straight press on any given night...gameplanning options are fun to have!), but finally ditched the zone component and went straight press for the added turnovers it created (the new engine switchover, in my opinion, really neutered the combo defense vs. what it had been old-engine). The 20 minutes I was throwing into the zone half of things seemed better used by bumping the press from 20 to 25 minutes of practice time and lacing the other 15 minutes into individual practice plans (and the press required more bench minutes, so my younger players improved slightly faster due to more minutes played as well under the press...).
Can a combo still be run well -- absolutely. In addition to the attributes mentioned by some of the above (the REB and SB are an absolute must in your bigs if running zone/press), I would also add that you need to pay greater attention to WE than you would in most other defenses. Since you're almost forced to sacrifice individual minutes (or risk not putting enough minutes in team practice to raise IQ's all the way, which I would say is a bad way to go), you need guys who will reach their max potential with reduced practice time.