The UI in their partner portal. It's very cumbersome. As an example, to request a transfer authorization code, you can't just bring up a domain and request it. No -- that option is nowhere to be found when you bring up a domain. Instead, you have to go to Search > Advanced Search, search for the domain, and from the search results page select "Advanced DNS" from a drop down and click on "GO!" under "Manage Multiple Domains", then check the box next to the domain, select "Request Auth Code" under another drop-down, and click "Go". It makes no sense. But that's even how they say to do it in their documentation.
It also has not changed a single bit that I can tell in the many years we've been using it. It's not at all mobile friendly, and even on desktop it doesn't take full advantage of the screen space.
Registering a domain for a new customer is also a hassle, and all search forms are over-complicated. You have to select a radio button next to what you're searching by rather than just entering a value, and you can't search by more than one thing at once without using multiple forms. Review collected by and hosted on G2.com.