It's not mentioned in the release note, but I find implementing "Mix commit notes with merge request notes on MR show page" (https://github.com/gitlabhq/gitlabhq/pull/1630) the greatest change in 3.0. We are using merge requests as a tool in code review, and this commit finally allow us to go through all comments of an MR in a single page. It would be perfect if the issue "Save few lines of code with Merge Request comment like github does" (https://github.com/gitlabhq/gitlabhq/issues/1007) is implemented, as this would allow us to finally reliably comment directly on the "diff" tab.
[1]: https://www.synalysis.net