I was wondering if a non-drummer (but a keyboard player and midi progammer) could get in this discussion.
I once heard Quincy Jones talking about songs being "in the pocket." He said it was about tempo, shuffle, and timing, and he wasn't just referring to the drums.
To me, a beat is "in the pocket" when:
1) the tempo is just right. The songwriter may have written the perfect song, but now its up to the band to play it well. So the drummer starts playing it at 95 bpm, like he was asked to. But then, as the band continues practicing the groove, they find that it all flows better doing it just a tiny bit faster, say 96 bpm, or even more precisely, 96.5 bpm. It sounds weird, but getting the groove at just the right tempo can put it in "the pocket"
2) the shuffle is just right. I'm a big fan of both jazz and hip-hop, and my favorite hip hop beats use strong shuffle. Now, traditional jazz shuffle and that hard hip hop shuffle are about 67% shuffle (a "straight" beat is 50%). However, its up to the rhythm section (particularly the drummer) to find the perfect shuffle for that song. Through experimenting and practicing with the band, he might come to "feel" that a 57% shuffle is just right. Not only that, but he "locks" that shuffle in his mind, and does all his fill-ins and cuts according to that precise shuffle: in other words, he plays disciplined. That's a part of playing "in the pocket"
3) the timing is just right. through experimenting, he finds what notes to play slightly early/slightly late to get a nice feel. You all, of course would know about this more than me because you all are drummers. But I think its important that whatever scheme of early/late playing he decides on, he plays that consistently through a song, to keep that "pocket"
4) the volume control is just right. if the drummer decides he's gonna play quarter notes forcefully and sixteenths notes softly, almost like ghost notes, keep that throughout the song, except on parts where you're doing something special. That's the pocket.
To summarize: when one intelligently, thoughtfully combines these ideas, and plays them with discipline, that's POCKET. This is why drum machines and computers became so popular in music, especialy with the urban crowd, cause you tell the computer to play that sound and it faithfully plays it throughout the song.
Anyway, just my .02$
