Most of the issues I have with Gitlab's UI are the result of many small, seemingly insignificant and probably nitpicky-sounding choices that tend to build on to one another. All of these are my humble opinion:
* Everything about browsing projects. Okay, so if you click on "Projects", you get taken to a list of "your projects". That's kind of annoying since I'd go to my personal page if I wanted a list of stuff I've touched (kinda like Github) but it's defensible as a choice. If you want something that isn't yours, it takes another click to "explore projects", which goes to a "trending" page that will either be blank or have very few things on it, and another click after that to get to the time-sorted list of everything based on last update, which is usually what I want.
* Groups appear to be second class citizens of sorts, rather than the top level organizers they really are. (I.e. groups are separate from projects, even though all projects live in a group or user space). Bitbucket did this better - when you log in, you see a list of groups first, and from there drill down into projects.
Basically, it kinda wants to be a hierarchy but offers too many ways to circumvent it. Imagine if you were just browsing a file explorer and saw files two levels below your pwd. It's disorienting.
* Top level project pages feel cluttered and noisy. Github does this better, without requiring a separate sidebar.
* Repo settings are buried under expand links. This did not make the page more usable, it made it so I had to guess where the setting I want is.
* Why are there two nearly identical pages for showing repository contents? (One at the top level of a project, which is any project's landing page, and another on its "repository" tab, which is the same view missing a few elements)
* Releases and their artifacts are buried behind "tags" (which they technically are in Git parlance, but still) or a very easy to overlook CI status badge on the last commit shown on the landing page. Releases can only be created with API calls or manually through the new tag page, rather than programmatically or as part of a CI job. (Yes, I know, the CI job could technically call the API. You get a cookie. Point is, it feels rather buried.)
* Build artifacts can only be downloaded in a zip bundle, rather than individually
* I haven't figured out how to turn off the "Do you want to enable Auto DevOps?" div showing in every single repo.
* Advertising for Google Cloud on the CI Kubernetes page is just plain tacky. I'm sure there's a way to turn this off, but I shouldn't have to in the paid Enterprise product.