Things that make me not use gitlab (some of them are super shallow, stupid and relatively unfounded):
1. They used to use Azure, so even though I know Azure is actually an awesome cloud platform, I get this mental image of "Microsoft MVPs" constructing gitlab using VBScript macros because I come from the M$ era.
2. the very public data loss that happened a few years ago, that sounded like a mess, gives the lasting impression that repos are not lasting if they are in gitlab.
3. I never knew gitlab didn't just look like bitbucket, which I hate. In fact, I never would have looked at all if I wasn't writing this response, so I guess to get me they would have to lead with "Nothing like bitbucket!!"
4. I've spent so much time in github, that anything even slightly different pisses me off.
5. The logo looks like firefox porygon edition. GitLab is such a cool name, it could have a better logo, like the word "Git" followed by a chemistry beaker / "potion" thing.
6. The only marketing that would ever work on me would be "has an interface identical to github, does all the things github does, plus these 30 other things". Most of their marketing leads with the "30 other things", so I click away before I bother to read it.
7. If GitLab or anyone tried to do something innovative like writing an extended version of git that adds comments and pull requests to the underlying CLI, and bakes them into the repo itself in some way so we can just stay in CLI world all day, I'd be absolutely onboard, but no one does that for some reason even though that would be a huge differentiator. A similar strategy worked for Heroku if you think about it.
8. Every single open source project I use or contribute to uses github. If GitLab were to, say, add a github integration that lets me work with all the github repos I care about, but using Gitlab's UI, I would be intrigued.