1. If the singing group is all women, be sure to say they were "sassy but classy".  In fact, say it two or three times.  Unless, of course they weren't, in which case you tell them that you wished they could have been more "sassy" and/or "classy"
2. Make a reference to the fashion of the era of the song.  For example, if they sing something from the fifties, say how much you wanted to put on bobbie socks and go shagging.  If they sing something from the seventies, say how much you wanted to don an afro and boogie down on the disco floor.
3. Song titles are easy to incorporate in your criticism, because you can figure out in advance what you will say.  For example, if they sing "Come Sail Away", then you can say how they really did take you on a boat ride.  If the song title has any fire or heat references, you can tell them how HOT (get it?) they were.  Puns, as we all know, are a sure-fire audience-pleaser. 
4. Similarly, always tell that Latino group that they "took you" to Puerto Rico.  They never get tired of hearing that.
5. Don't invite comparisons between you and Paula Abdul.  For example, show up sober.
6. Don't engage in any legitimate music criticism as if you were someone scholared in serious music theory.  Ben Folds will show you up and own your ass.  Come to think of it, so will the Boyz To Men dude.  Let's face it — you wouldn't know a tonic note from a gin-and-tonic, so just stick to how the song made you feel, because nobody can dispute your feelings. 
7. "You rock" works too, whether you say it once or twenty times.  If you're comfortable with the idea, you can modify the phrase a little, as in "you guys rock" or "you were really rockin'".
8. But be sure to leave the really hip language ("that was fly", "that was dope") to the Boyz To Men dude.  He can get away with it better than you can.
9. A little self-promotion of the Pussycat Dolls here and there won't bother anybody.  God knows you need it.
10. If you want to sound clever, refer to it as someone's "instrument", not as someone's "voice".  As in, "Melanie, you have a lovely instrument and it rocked".  I know that's confusing because we're incessantly reminded that there are no instruments (really, there aren't any at all), but in this limited instance, it's okay to use that word.
11. Other phrases that work, provided that Ben Folds or the Boyz To Men dude don't say them first, include: "You made it all your own", "You deserve to be here", "You're already winners", "This is what a cappella is all about", and "I can't believe there weren't any instruments".  Note that these phrases also work particularly well when you want to say something nice following a less-than-stellar performance. 
12. You can also eat up your alotted time by adding "…am I right, audience?" to any of those phrases mentioned above.  It's a virtual lock that 5 to 10 seconds of audience applause and "woots" will follow.  Just be sure not to start off your criticism with "Am I right, audience" — it has to come after you've used one of those phrases.

I've been a fan of college a cappella for the last ten years.  Of course, I was a fan of the Bubs when I was at Tufts (it's required).  It was an interesting time in the evolution of college a cappella.  The Bubs were already considered the best a cappella group among the New England college circuit (and in New England, there's a college every half mile, so that's saying something).  But during the time I was at Tufts, the Bubs were taking the genre to a different level. 

Prior to that time, most of the college a cappella groups, the Bubs included, had a repertoire that included traditional standards.  The most "contemporary" anybody got was "Happy Together".  The Bubs had already infused their act with humor and charm, which already set them apart from, say, the stale Yale Whiffenpoofs, but during the time I was there, they started adding movement and contemporary songs.  Marti, a friend of mine, was the Bub president, and I remember him talking about doing "Thriller", something unheard of.  And not just the song, but the dance as well.  He did it, and it was a hit not only at Tufts but at other campuses where the Bubs toured.  And that set the standard for the next couple decades (I worked with the Bubs on stage movement here and there).

But once I graduated, I never paid much attention to college a cappella until about ten years ago when I happened to hear the Bubs's studio recording of "Owner of A Lonely Heart".  And the innovation there was the "mouth percussion" which added a whole new dimension.  There was also overdubbing and a few electronic tricks which made the studio recordings simply unbeleiveable.  I doubt this was a Bubs innovation, but I dug in a little more and realized that most college a cappella groups were doing, and the sound was amazing.  And soon my iPod was full of the stuff.

I've always wondered what the reaction would be if college a cappella went mainstream, so I'm happy for "The Sing-Off".  Of course, that show embraces all kinds of a cappella, not just the "contemporary college" kind.  But I am glad to see it is out there.  And I'm glad to see that the Bubs are still considered among the best of the bunch.  There's a lot of good groups out there.  The Socal Vocals (not to be confused with their alumni group, which is on the Sing-Off) is consistently good.  University of Michigan always has a good sound in the co-ed category.  For all-women's groups, the UNC Loreleis actually is a fairly strong group.  BU's Dear Abbeys, an all-male group, is currently my favorite.

The problem with ALL these groups, and The Sing-Off's presentation of them, is that they simply cannot replicate LIVE what they do in the studio, where overdubbing and remixing ensures a pitch perfect end-product.  But for those who are curious, iTunes and many websites have good samplings of outstanding recordings.