An article about immutable data structures (which is an excellent read in itself) has two interesting paragraphs about Git:
Casual observers initially criticised git for having a model so simple it was actually naive. It turns out they were confusing the model with its on disk representation. Git makes this distinction very well, and the result is that it implements powerful features (for instance idempotent patch application) which are apparently too complicated in other systems. Git itself isn't simple at all, the problem of version control is a complicated one so any system dealing with it is inherently complex. Git's advantage is that it's built on a very simple and future proof core, allowing the complex parts to evolve more easily.
A very nice way to put it, Yuval.
I've linked this excerpt differently. The linked Git documentation file is well worth a read, if you care about why git turned out as it did. Not that much interesting for day-to-day usage though.