A couple of things. It was proclaimed as a celebration of how unique everyone's channel was, when it really just took away all but the shallowest of customization. Channels now looked a lot less distinguishable. The immediate recognizability of many channels was discarded for one (very weirdly sized) banner, and that's about it. Perhaps not the biggest problem, perhaps a favourable change to many, but it was not popular amongst creators.
It also made a lot of partner perks redundant. Before, it used to be the case that you needed certain numbers of subscribers and views to earn the ability to upload banners, create thumbnails, add links to your banner, etc. It encouraged quality, audience engagement and platform loyalty to grow your channel. With it gone, what could networks offer to entice creators? This was, if I recall, near simultaneous with YouTube allowing
anybody to monetize videos, not just partners. Which is a lovely idea, but the total ad prices didn't increase. Everybody's joint earnings were spread thinner, mostly amongst smaller channels earning too little to ever actually claim it. How unfortunate for YouTube to have to be holding all of that cash for them...
The bigger problem, though, was that it stopped audiences from viewing content on a channel's home page. It added an extra page load to watching the latest content (and when you're designing user interfaces, the rule of thumb is that each page halves your audience), but even worse was that it discarded a whole metric. The amount of video views on your home page was a major calculation in your earnings, and without it everybody's earnings plummeted. There was no warning. The networks had no immediate solution. Everybody had to switch back to the old style or go broke, knowing that snowflake would be mandatory soon. I believe they fixed this at least.
What Varrok said, coupled with a slew of bugs. You can't reply to comments left before the change. When you delete a comment, it deletes everything classed as a reply whether you want it to or not. Sometimes none of the comments that are left will show up. It's screwing with our audience engagement and we had no choice or forewarning. But bugs can be fixed. Bad design can be fixed. There are some nice features. But you have to be on Google Plus, because Google gots to have the big data on you.
YouTube co-founder Jawed Karim left his
first comment in eight years in response to the new comment system.