I like quite a few things about the 5E Monster Manual.
But, there are a few minor things that I dislike:
1) Light blue colored background that has the equivalent of ink smears on it. It makes the pages look dirty. The light colored background in the PHB is similar, but the difference is black print on a contrasting color in the PHB and the "colored smears" are so feint as to hardly be noticeable. There are some pages where black on blue (or feint black smears) in the MM just make it more difficult for my old eyes to see the text. It's legible, but it's harder to see than it should be. I prefer to not have to squint a little in some places, just in order to read. I do not have this problem with the PHB.
2) No list of monsters by CR. There's a web printout, but it's annoying to have additional sheets stuck into your Monster Manual that will eventually be misplaced at just the wrong time. It's also supposed to be in the DMG, but until we get that, ... Like the spells not being segregated by level in the PHB, this just seems like a usability issue. If I go out of my house and want to set up some encounters, I now have to take two books.
3) No table of contents of the appendix creatures or of any of the specialized creatures (like a Blue Dragon). Again, a minor nit, but when the DM is going "where is the horse, where is the horse?" because it is not in alphabetical order. And the Index is partially a repeat of the Table of Contents, hence, waste of space. Sure, the exact page of a Black Dragon is listed in the Index, but it's not too hard to find a Black Dragon by looking under Dragon. The Index almost seems superfluous. So to find a Gargoyle, the DM can look in the easier to read (larger print and a single page) Table of Contents. To find a horse, the DM either has to know which appendix it is in, or go to the Index. The DM also has to know if it is a riding horse, or a warhorse. Horses do not exist. It just seems that they could have put the entire Index in the table of contents and been done with it.
4) The pages are wavy (i.e. if you close the book and view it sideways). Not a biggy, but just subpar for a brand new book. This might not be true of all MMs.
5) I'm also not a fan of the "weird aura" artwork around creatures that most of the monsters have around them. It just detracts from the picture. Sure, there was some of this in the PHB, but not a lot. Here, it's practically every picture. It makes them look like good artwork superimposed on bad watercolors. I'm sure that some people love this, but meh.
Like I said, these are minor issues (and #5 is totally subjective). I do like the content and some of the art.