According to Panasonic very little except the box they are in plus "minor contrast changes" - well thats what they said for the TX28PK25 and the PL10 which are basically the same but a 100hz.
Yes, I ordered a PL10 and when it arrived just wouldn't start - lots of clicking but no sound and no picture. I got a PK25 instead and very happy with it. To be honest, apart from the stand, they are very, very similar.
If they had had a PL10 in stock though I would probably have gone for it because it is the newer model, but thats the only reason.