Well, the term "underrated" can mean different things, obviously. In the context of the original post, it seems to mean underrated by the general public and not necessarily by critics. For the purposes of this discussion, I would operationally define "underrated" as either a record generally regarded as a "lesser" record, either in sales or in the estimation of critics. For example, I regard Fleetwood Mac's 'Mirage' as an underrated album. It didn't sell as much nor was it as well-reviewed as 'Rumours' or 'Fleetwood Mac', but it is better, in my opinion, than either its sales or reviews indicate. As mentioned earlier, Led Zep's 'Presence' falls into this category (not so much for me--for me, their third album is the charm, although I'm not sure I see it as underrated).
Then there are the bands that are critical darlings, including many of those already named. In this context, I think "underrated" applies to very good or possibly great albums not as highly thought of by the critics or the band's/artist's cult following as other albums.
As completely random examples, I would choose REM's "Automatic For the People," Television's "Adventure," and perhaps Miles Davis' fusion-period albums ('Dark Magus,' 'Black Beauty,' etc.).
At least it's easier for me to think of it this way. We could name underappreciated albums all day. I don't see that as the same thing as underrated. But I wouldn't want to be forced into debating the merits of my case. :-)
