1. Complex information model
The information model is complicated – and you need to know all of it. As a point of reference, consider Subversion: you have files, a working directory, a repository, versions, branches, and tags. That’s pretty much everything you need to know. In fact, branches are tags, and files you already know about, so you really need to learn three new things. Versions are linear, with the odd merge. Now Git: you have files, a working tree, an index, a local repository, a remote repository, remotes (pointers to remote repositories), commits, treeishes (pointers to commits), branches, a stash… and you need to know all of it.
(Full Story: 10 things I hate about Git « Steve Bennett blogs)