It's secure (always escape content (including attributes) unless you resort to use raw response writer
Certainly many (if not most) templating frameworks (Jamon, Mako, etc) do this. Nothing novel there.
IDE/refactor friendly (pure java, live html help/hints without plugins);
While that's certainly true about Hamlet, I question whether or not that's a goal which warrants writing/maintaining a brand-new templating framework.
Statically validates html (you can still generate bad html if you really try but you can generate valid html much easier).
Certainly there exist templating frameworks which can do this, or at least we could wire in one of the many HTML validators out there to do this when running the tests. This is what I've done in previous web projects with great success. Again, this feature doesn't seem to warrant writing/maintaining a brand new templating system.
Since you don't have to learn any new syntax (like wtf is <%& etc. in a template system)
You don't have to learn a new syntax per se, but you do have to learn a new (and IMO arcane) Java library. Like for example, what's with the "$style" method? or the many calls to "_()" all over the code? These were questions I had when first looking at the code which uses Hamlet, and I'm guessing Bobby had the same sort of questions. By using Hamlet, we're cutting off those developers who do have some experience with existing templating systems in favor of a new system which literally no one has experience with. As someone who's used several templating systems in other projects before, I can say with confidence that being required to use Hamlet will make me less productive than a traditional templating system would.
I don't want to keep harping on this, as I don't feel super strongly about it, but it still seems like a mistake to me to be introducing a novel web templating framework into Hadoop. The Hadoop project, IMO, shouldn't be in the business of web templating. Rather, the Hadoop daemons just have a few web pages they would like to display. Why should we re-invent this wheel which has already been invented many times before? If I'm the only one who feels this way, then I'll shut up about it.
I know HBase uses Jamon for rendering its web pages. Perhaps someone with experience developing those can comment?