Usually, there's about 3 main characters, which have their own little subplot as filler for building into the main plot. But today's franchises take that subplot and continue into that character's own film, which gives us the "ok what's next after this" syndrome, instead of enjoying the story we have playing on screen in front of us. Too much plot is when one piece of the puzzle is only linked to yet another piece and we never get to the end climax because of so many hoops the characters jump through, an by the end, the viewers are just tired.
I like simple plots, but i HATE spoonfed stories. If the movie is a mystery story, then the viewers need to figure stuff out on their own, I always see blogs and articles spoonfeeding the answers, but that takes the fun out of those kind of movies. It needs just enough to keep the viewer intrigued and interested, too much and the viewers can be lost. Action is always good, but not the whole "we need you to get them and they need to get the special tool that will get us halfway to keeping this disaster from happening, and that tool will lead us to the other guy who knows the people that will help us win this" The second and third Matrix films had that going, Ultron and Furious 7 kind of had that going on too.
All in all, overexposition to explain everything is just lazy, and overcomplicated plots are just too much for people to process.