Yes, STASH-2550 is the open suggestion that covers improving this feature.

This feature seems to be unable to compare arbitrary commits, only those which have tag or branch referring to them

Ah yes, another misleading thing with tags. It's like the opposite of the branch create feature. When viewing a tag you can create a branch from it but it won't actually let you create a branch from the tag.
Atlassian really needs to get their tag support on par with the rest of Stash.

Thanks Kyle, when looking at the Commits view for the repository, the dropdown when viewing a tag does not offer the "Compare" option. So it was misleading.

You can compare tags. On the branch compare page choose the branch dropdown and change to tags.

A bit annoying that branch comparisons were introduced but not *tag* comparisons. They're functionally the same thing, there's no reason why the interface shouldn't allow it.

Stash 3.0 now includes branch compare functionality.

To use it, either click on a branch name from the branches tab, or choose "Compare" in the branch menu that appears on the Source, Commits and Branches pages.

From here you can view the unmerged commits and changes between branches or tags, and then create a pull request. We are planning to improve the process in a future release to show diffs when creating a pull request (watch STASH-3290 for updates).

+1

+1

Agree with Andrew Herron. The brilliant diffing screens (compared with FishEye) is one of the killer features that caused us to move to Stash, but it is a shame that we can only view them in pull requests.