To me, the biggest problem with ROTJ is the gargantuan plot holes. In particular, the rescue plan is incomprehensible, and the emperor's plan to turn Luke makes little sense (particularly since he is telling Luke his plan).

However, it is still a really fun movie and the two new characters (Jabba and the Emperor) steal every scene they are in. The climactic scene with Vader (sans "no") is terrifically done. The ewoks are fine - there could certainly be creatures like that on some alien world. I still think it's an excellent movie despite the flaws... given the direction ESB took the "saga", ROTJ was an almost perfect wrap-up IMHO.