>Is it possible to set up protected branches, where I can say, only users X and Y are allowed to push to the master branch?
Yes. I recommend (and my employer uses) a Herald rule that admits pushes to master only if the changes are in an approved Diff (unit of code review). This way you get an enforced 2-man rule, but anyone besides the author can approve the change.
This is currently impossible with Gitlab AFAIK, and is the main reason I won't use it.
You can also configure Herald rules to add blocking reviewers to other people's Diffs based on arbitrary criteria (i.e. touches X file and it's a Tuesday). A Diff cannot be considered approved until all blocking reviewers have signed off.
Reviewers can also be groups, so you could say, for example, changes that touch Y file must be reviewed by Security, but any member of Security can sign off.
>Can I do a code review and then finalize it when I'm finished and does the author get notified with a summary like the new GitHub code review feature?
Yes, everything you do on a Diff is batched and happens all at once when you click "Submit" at the bottom. Inline comments, the free-form text box, and the action all come as one email.
> What I really like about the merge/pull request workflow of GitLab is, that multiple developers can work together on one feature branch while a merge request is open and can be used as a discussion platform for the changes - is this possible with Phabricator?
A Diff is really meant to be an individual's small, well-defined contribution. Multiple people submitting code to a one diff can get messy. This would be better modeled as many diffs.
You are allowed to land diffs against branches other than Master, however this gets messy. It is not a first-class use case. My employer does not collaborate on feature branches; instead we land half-finished features into master (and production) disabled by feature flags.
It is NOT a merge/pull-request workflow. It treats Git more like Subversion; Phabricator is a very sophisticated alternative to emailing patches around before SVN committing them (basically the equivalent of landing).
>Also I'm wondering how solid the issue tracking is, and if the project workboards can compete with Trello.
Issue tracking is totally solid, very configurable. Project workboards consist of tasks/issues, which are much more expensive to create than Trello cards: you fill out a pagelong form, pick type, severity, owning team, title, description, etc. vs just free-forming a title. However, you can drag tasks around the workboard just like Trello and it looks pretty similar.
>Can it be used by non-developers without giving them access to the code?
Yes. Permissions (and approval flows, and notification rules) are very configurable. Phabricator is well suited to a large organization with rules, but this suitability doesn't carry too much overheard.
I don't think you could collaborate on issues with customers. Nontechnical people in your company, though, totally - we do that.