There are two aspects here:

One is the visual design, and even the author agrees that it's probably too similar: http://news.ycombinator.com/item?id=3744356

The other is the idea/concept. Creating open-source versions of unique ideas is nothing new and I doubt you'll see much sympathy on this front.

I'm wondering how is this very different from MS Office vs. LibreOffice...

Granted, latest incarnations of MS's products are a bit different with the (crappy) ribbon and all, but LO is quite similar to previous versions of Office.

And I'm talking both about UI, design and functionality. Think Excel/Calc functions, for example.

Right, and as Mr. Curtis said, the author copied his design work. Mr. Curtis isn't crying fowl that someone built an open-source blog engine.

If you copied the articles, yes. If you copied the design using none of their assets or code, no. Copying it definitely is, stealing it definitely isn't (at least not in the legal sense, the moral sense is debatable.)

You should have confidence that you will win because you can implement it better, understand the need better, can craft better solutions faster, and have better content on your network. If those things aren't true and all you had was an idea, unfortunately this was bound to happen.

Ideas alone are not defensible, practically or even legally. Successful implementations thereof can be however.

Copied your work. It's ridiculous, of course, but you won't win any fans here with linguistic imprecision.