Yeah maybe that's it - it's like an older ROTJ Hamill. And yeah, I have it on the custom below (I replaced the head on my non-Bd Bespin Luke with the newer Jedi Luke version but ESB hair, next pic down) and it always strikes me as just a little underwhelming, hard to put my finger on why. To me it's a more "male model" Luke, too chiseled and handsome.
I get what SS17 is saying, and I don't see the newer Jedi Luke head as a bullseye (slightly weak jawline for Hamill on that one, and also works better for ROTJ than ESB) but the DX07 non-BD just feels a tiny bit meh when I look at this Hoth base custom, and I've seen people install it for a custom Hoth Luke and I still don't quite love it.
It's not a great pic to illustrate it, but the paint quality difference on the DX07 vs the newer Jedi Luke is fairly noticeable. But as I said, still defineitly able to be displayed alongside newer HT stuff.