a) Lots of dead space. I estimate that some of their commentaries include as much dead space as actual talking.
b) Lots of laughing at jokes or "Oh, I love that" type of lines.
c) Occasional confusion as to what episode they're watching.
d) Almost no actual information whatsoever, and any information they actually do give is repeated in the "inside look" featurettes.
Now, you might plausibly point out that most of the "Seinfeld" DVD commentaries share these features: the commentaries with Jason Alexander, Michael Richards and Julia Louis-Dreyfus are the same way, and so are the commentaries with the "other Larry," writer Larry Charles (who's sort of a hippie version of Larry David, both in appearance and in writing style). But something like the Seinfeld/David commentary on "The Opposite," on the upcoming Season 5 set, is truly a masterpiece of vapidity. Sample lines from this commentary:
LARRY: What show is this? This is the end of season what?
JERRY: The funny way he prounounces "parents."
JERRY: That's one of my favorite lines.
LARRY: Mine too.
JERRY: It makes sense to him.
LARRY: Makes sense to me too.
LARRY: Boy, that was a great show.
The sad thing is that I think I've just transcribed the bulk of the commentary.
Admittedly, a lot of TV-on-DVD commentaries are a little like this; there's not a lot of time to talk over them, and TV production is so fast that it's hard for the participants to remember what went on in a specific episodes. (One of the reasons why a show like "The Simpsons" lends itself to audio commentary is that an episode takes nine months to produce, which perhaps means that the participants can recall more details of the production.) Still, the Seinfeld/David commentaries are kind of in a class by themselves: they sound like two guys who have been locked in a room with a bunch of episodes and are just watching the show and making occasional small talk until they can get out again. Which might, for all I know, be the case.
By the way, the season 5 set -- I haven't seen season 6 yet -- is excellent, as long as you stay away from the commentaries; the "inside looks" provide more information than the average commentary anyway. I particularly like the collection of promos for the show's move to the time slot previously occupied by "Cheers," which are relaxed little scenes done in the style of the show, instead of the high-pressure promos we'd expect from a major network today.