A guy that normally plays barbarians in the one game I'm playing in is trying out a monk and has gotten into trouble because he keeps trying to tank on a class that's built as a skirmisher. It seems like a lot of people fall into that trap.
"When all you have is a hammer, every problem looks like ... SHOOT IT WITH ELDRITCH BLAST!"
-This guy who likes playing Warlocks, probably.
Fundamentally, the issue with the Monk is, to paraphrase the late, great, Steve Jobs ... the people who don't like it are holding it wrong. Simply put, monks don't play like other classes.
They have the following issues which make them really stand out:
1. They are one of the "short rest" classes. So there are many people that complain about ki, when really the issue is the table's use of short rests.
2. They don't have spells (not entirely true, given a certain kludge-y subclass, but true enough). There are a lot of players that need the crutch of spellcasting; there's a name for those people, and that name is Bard-lover.
3. They don't fit the typical archetype of the non-spellcasters. They aren't skillmonkey/glass cannon rogues, and they aren't FIGHTY McFIGHTFACE, and they aren't Conan Sir Tanksalot.
So what you have is a class that is
sui generis. It isn't like any of the other classes, and it doesn't fit into any neat category. A lot of the abilities don't easily "math out" for optimization purposes- like being able to avoid falling damage, or spells, or missile weapons, or the crazy mobility that they have. Some high-level abilities are just insanely good, but rarely taked about (proficiency bonus on ALL ABILITIES?).
There's just a lot of weird buttons to push, and most people aren't used to pushing them. It's not a class for everyone, but for some people, it's really enjoyable.