I checked my disk brakes for the first time last night (i've never had disk brakes before). The front ones were definitely worn out, and I replaced them. They work great, now.
With the rear ones, I decided that they still had quite a bit of life left in them, and put them back in, rather than replacing them. Now, they hardly work at all. They were working fine before.
I think that I was careful not to get them dirty while I took them out and looked at them, but I guess that it's possible that I got some dirt on them while I took them apart and inspected them.
The other possibility is that maybe the pistons which push the brake pad onto the rotor got pushed back too far out, when I was putting the pads back in, and so now they're not pushing the pad onto the rotor with much force.
Is this a known gotcha?