When I see Phabricator I just don't see anything. I just see a blob of text dumped to the screen. Maybe it's because I'm so use to Github.
It's the little things that GitHub does, that I've found makes the difference. Take GitLab's branches page here:
https://gitlab.com/gitlab-org/gitlab-ce/branches
vs
https://github.com/gitlabhq/gitlabhq/branches
By limiting the number of branches shown and by creating strong visual breaks/barriers, they make it way easier for the user to focus. Seeing a wall of branches is okay ... when that's what you want, but as a default, by organizing it the way that GitHub does, they make their branches page significantly easier to consume.
I also find the buttons too be a little too big in GitLab and they don't have enough definition to them, to help you focus on them. Creating merge requests and comparing branches is the focal point of the branches page, and they should make it easier for your eyes to lock onto them.
Like I said, it's the little things that I can't explain that I've come to respect and I don't think it's just because we are familar with GitHub.
I really like what GitLab is doing, but I also really like a powerful web-based repository browser (search tool included). Hopefully they just went a bit wild with the UI and they'll rein it in. (I also checked Preferences and they have a switch for fixed vs. fluid layout, so maybe they've been responsive to UI-change revolt in the past?)
The switch was surprisingly controversial, as the GitLab network view was apparently much more useful than the Github network view.
GitLab just needs to work on the little things and they can really be disruptive to GitHub's business.
Another little change that I think would go a long way, is changing the folder icon. I don't know why, but the round corners in the folder icon really irks me. There is also something off about the font that they are using. I think it's just too thick, but I can't really put my finger on it.
However phabricator maintains different "applications" - bugs, repos,etc. so to look at the bugs for a repo, I have to go to the top level, click on Maniphest (the bug tracking "application" ) and go to the project and then click on a bug. very unintuitive.