Call them what you wiill, but the flaws that are usually in sequals, most notably extending a story past it's dramatic/emotional conclusion and having to invent a new conflict, are absent from LOTR due to it's nature as being three parts of the same story (which is also how the novels were published, due to paper prices).
SW, ESB, and Jedi are not three parts of one single story. They weren't concieved that way and each is a stand alone story.
I agree that Sequals have the potential to be great. X-Men 2 is one of the best movies ever, built on the back of the somewhat pedestrian X-Men. It's just that the vast majority of sequals lack the dramatic spark (and the advantage X-men had of picking the best bits out of 35 years of published material).