I've figured out what it is I like about these: smooth upper mid-range. Based on personal experience (and supported by Tyll's
cheap headphone frequency response round-up) I'd say that this area of the audio spectrum tends to get neglected somewhat. It seems that manufacturers generally want to make sure that the treble is strong, and bass rountinely seems to be overemphasized, but the upper-mids are given short shrift by a number of otherwise good cheap cans, such as the portapros and Senn 497s.
The ATH-M30s are quite smooth through the top of the mid-range, but treble is rolled off somewhat. Vocals dominate the presentation in rock/pop, jazz, and even classical (oratorio, lieder, etc.) Upper bass is overemphasized, but not in that "big bass" way that some phones seem designed for. The overall effect is somewhat laid back, but I wouldn't call it remote except on a minority of recordings that I've tried (and these were tending in that direction anyway). Isolation is decent -- same general range as the Sony 7506, which these closely resemble in physical construction. The ATH-M30s are quite comfortable - the pads seem much softer than the Sonys, and pressure on the ears is moderate.
I wouldn't give these things a rave review, but I think they are worth considering if you've only got $50 to spend and you want a set of sturdy, comfortable, isolating closed cans. And I think they fit reasonably well the requirements I had in mind when I started exploring closed headphones, which is something good for long-term listening in environments where I want a little sonic privacy while reading. None of the others I've tried have worked as well. The Sony 180's and Philips 555 don't provide much isolation. The Senn 202s are better in this regard but not great, and I find their sound simultaneously boomy and aggressive -- I like it in certain moods, but not for background listening. The Sony 7506s are great, but again don't work for me while I'm reading. Their presentation is quite forward and, for me, hard to tune out -- great for critical listening. So I guess the ATH-M30s will do for now unless anyone knows of something similar that costs less.