1. It's no longer regulated by actual human beings, just an automated system.
2. The entire comments section is not organised, lost it's dislike button, and is filled with people less capable than a drunk pop tart. Just a mess.
3. By far it's biggest problem;
how easy it is to abuse the copyright system, whether it's those reaction channels, or fake claimants. The fact that YouTube allows fake claims free passes to steal content creator's money is insane. This can be fixed if competent people are hired to manage the community, but, as you can see from the first problem I've mentioned...
4. It's awful "guilty until proven innocent" philosophy, regarding copyright issues.
5. Prioritising views instead of actual video quality (although all business is like this, so I can't blame YouTube on this one. Still kinda sad, though).
6. Not taking care of the community. It's letting prank channels and vine compilation channels post abhorrent content, some of which (especially the prank channels) show these channels having no regards over simple mannerisms, personal space, and humanity as a whole. Disturbing and irritating people, disrupting the peace of society just for views. I'd honestly rather see fake pranks, cause at least I know it's not actually cancerous to real life (although they're still awfully desperate). The lack of control is giving content creators the impression that, as long as you get the views, YouTube won't care if you harass people for your videos. FOR DEM VIEWS, AMIRITE?

DD
Let's Play video gamers, vloggers, rant & reviews channels, whether you like them or not, aren't the problem. Far from it, they are the sort of people who make YouTube what it is. Pewdiepie might be annoying, but
he makes his own content, he doesn't go around town harassing people for views, or putting up false claims to steal other people's money. Just like any other legitimate YouTube celebrities (heck, even The Fine Jews were legitimately good content creators until the money and the fame got way over their heads).