A good UI exposes the underlying structure where necessary, while offering shortcuts for commonly used tasks or groups of tasks, and most importantly of all, it is consistent. The internal design is elegant and simple, but the UI has grown organically and does not show the same level of care. I'm not convinced the problems with git lie with some fundamental difficulty with its internal model, they are more basic and it simply hasn't had as much care put into the UI as it should. I use the command line, and off the top of my head, problems with the UI are:

Inconsistent options between subcommands, e.g tag and stash

Complex options for everyday commands, e.g. git reset HEAD --file, git rebase -i HEAD~3, git checkout -b mybranch

The mix of options and subcommands, with no clear rationale and unclear naming (things like reset)

Separation of the stages of adding files with committing, I think the default should be combined, with options to separate the stages if required.

The man page assuming understanding of fundamentals for simple commands like git push or rebase

Finally, as an example of how unintuitive the UI is around something as simple as reset/undo - here is one of the top stack overflow questions for git with 12195 votes, which just wants to know how to undo a commit:

http://stackoverflow.com/questions/927358/how-to-undo-last-c...

to which the answers are commands like:

    git reset HEAD~

    git reset --hard HEAD~1

instead of something like this in the default case:

    git undo