I have not seen the film so I can not say for certain what each of those criticisms mean. But from what I gather by reading the reviews is that;
- too much plot = too much time focused on little details, too much spoken exposition, not enough visual narrative (essentially the caveats and weaknesses of Nolan's involvement)
- too serious = virtually no attempt at levity for most of the film (which you have to admit films like BB and TDK had their moments) = not funny enough
But in all honesty, it seems like the illusion that this MOS is a great cbm/film and the hype surrounding it is being broken by a few negative reviews when it shouldn't at all. Comic book fans, fanboys and casual movie goers have the usual expectations of what they want from a superhero film with spectacle usually being at the top of the list. As far as I can tell, that's what they'll get in spades. But all I'm seeing are a few people trying to rationalize criticisms into overblown statements like critics wanting a campy film like Batman & Robin again. I don't think that's true. Critics have their own opinions; sometimes their opinions will align with yours, sometimes they won't.