Game quality and appeal is subjective. What is a fact is that they continually invested high amounts of money into projects which did not make a return. That's called "oh shit we suck at this and deserve to go out of business."
Most heard critizism about companies in general:
"Executive interference" and "pushed out games to early". Both which THQ didn't do.
Objectively THQ was very innovative (CoH and Metro eg.) with the games they published and gave the devs a lot of freedom. Too much freedom, obviously.
It was mostly bad management. And knowing Games Workshop they probably didn't make a lot of money on all the WH 40K games, which certainly didn't help.