I, like most of the TNG actors, lay the blame at the feet of Stuart Baird. The director is a big part of a films success or failure.
I'll have to agree with this to an extent. Baird's directing, or lack thereof, played a part in the failure that was Star Trek: Nemesis.
However, there's also the fact that by the time Nemesis came around (2002, I believe) Star Trek had been running continuously since 1987. In that time, six films had been made, three series had run with seven seasons, and another (Enterprise) was airing. Frankly, I think everyone was just about tired of Star Trek. The way I saw it, Star Trek really lost it's steam around Voyager. Sure, there were a few gems with Voyager like Year of Hell, and then there was Enterprise, which is really a mixed bag for me, but the fact is Star Trek was over-saturated.
And the fact that it was a blatant Wrath of Khan rip-off. From the damaged Enterprise to Data's 'heroic' sacrifice, the few gems that were in that film were blotted out by sheer repetitiveness, or down-right awful scenes.
Also, they sang.