I personally love Git. It is much better than anything else I've used before. However there is one feature I don't like. I'm talking about ability to merge branches.
You may ask why? Well, lets imagine a tree. Branches grow from it's trunk and never collide. Speaking in terms of graph theory there are no cycles. In Git you can create cycles by merging a branch into trunk (aka master) or into another branch. This is because Git represents history not as a tree, but rather as a structure called directed acyclic graph (DAG).
DAG structure for representing version history is common for all DVCS systems I know. I'm just providing Git as an example of the most popular DVCS system, and the system I personally use. All these systems call it a branch and everyone understands how it works. But in terms of graph theory it's not a branch, since you can create a cycle by eventually merging branch into it's ancestor.
Representing version history as DAG seems to be a design error, because it breaks logical flow of history and conflicts with the idea of incremental development. Advice: if you are aimed to develop software in the most logical and efficient way – avoid DAGs for representing version history. Just stick with normal tree structure and use branches only for release management.
I will try to tell more about this some time in future.