Sequels that are rehashes of the original plot. 80s Sci-Fi and action films are some of the worst offenders but lord knows some comedies have done this as well.

Adding a love story into a movie that doesn't need it. Especially jarring when it is in a sequel and involves the two main love birds trying to rekindle their romance, I'm looking at you National Treasure 2.

Making sequels/prequels to 10+ year old films. You may have made a masterpiece decades ago but trying to recapture that magic after so long just seems like a fool's errand to me.