I thought of another misconception, this time about TMP:
"Star Trek: The Motion Picture is too long, slow and boring. The Director's Edition fixes this."
I'll explain why I think this is a misconception that took hold.
I think this one is down to how most people (ie, "the broader viewing public" rather than simply the fans) experienced it on VHS. The theatrical cut of TMP has got its problems, including one or two pacing issues, but it isn't over long IMO. Its actually been cut pretty close, the only sequences that go on and on and on are artistic ones (and why shouldn't we see the full majesty of the inside of V'ger for the first time?).
What I think happened is this: a lot of people (again, talking about a broader public perception here) probably watched TMP on video in the eighties and nineties, and came to the conclusion that all the stuff about pacing problems was right.
But but BUT..... the version of the movie commonly seen on VHS was the Special Longer Version, a cut of the movie that deliberately threw in as many extra scenes and asides as possible, and was nearly 2 1/2 hours long. So, we got all these extra sequences of the crew reacting to things which just aren't in the original theatrical cut. We get many 'Keep The Camera Rolling' scenes where we pan slooooowly across the bridge, taking in each individual actor's reaction shot without a single cut or edit away. We're talking about two minute long scenes here. Repeated across the whole movie and all of that stuff adds up.
So yes, while TMP always had a 'reputation' for being long and unwieldy in places even in its original cinema exhibition, the SLV I feel only codified that plus moreso. People walked away making jokes about how slow paced the movie was. "The Slow Motion Picture", "The Motionless Picture", you've heard all the cracks. What isn't remembered is that the theatrical cut was something in the region of ten minutes shorter than the VHS version. It didn't drag in some of the places that the VHS one did.
And then the Director's Edition got plaudits for editing it down a further ten minutes to just over two hours. Loads of people threw acclaim at this, misremembering TMP as being this long wieldy beast and being thankful for a leaner version of the movie. But I think most of these people were comparing it to their memories of the SLV, not the already reasonably paced theatrical cut (IMO).