1. My grandpa the Marine says you can't polish a horse turd. This is apparently incorrect: you can polish a horse turd and sell viewings of it for $9/ticket in some parts of the country. It will, however, remain about as appealing as the unpolished kind.
2. If you are writing a movie, do not include a scene wherein the characters are forcing laughter at someone's pointless anecdote unless you are absolutely sure that the viewer will not think, "Oh, yeah, that's just like watching this movie!"
3. If you are an actress (or an actor, this is not a gendered thing), and if you have played a pretentious, self-absorbed, and emptily self-important character in the past, do not let those mannerisms and intonations slip into the character you are currently playing if she is not intended to be those things. We will begin to think that your part as the pretentious etc. role was perhaps not the acting tour de force it might have appeared.
4. Similarly, if you have played in an action scene before, try not to make movies where the exact scene is replicated by a different character, if no one seems to be aware of it. "He just does that thing she did in that other movie -- oh, yep, that's exactly what he did" is not so good.
5. It can be very good to undermine the viewer's expectations of a genre. However, if one of the viewer's expectations is "this will be interesting and/or fun," perhaps not such a good idea -- especially if you're not showing them why they shouldn't think the original is such a good time.
Hmmm. Perhaps this set of bad-movie traits is not as unique as it should be. Very well, then:
6. Remakes, possibly all right. Sequels, possibly all right. Sequels to remakes? Be very very careful.
And 7. If you are relying upon a previous movie or a previous segment of the current movie to make people care about the characters' relationships, you actually need to set it up so that they do care in the earlier work or portion of work.
Sigh. Back to the couch -- with a book this time, not a movie.