I honestly don't see Fillion pulling it off, either. And I did watch Castle, some, before I canceled my service.
If you'll all forgive me, I think part of it at least is that Fillion is the geek hero-of-choice. There's no denying the man has a strong fanbase of, well, us. Geeks. If Bruce Campbell were younger, or if we were discussing this all five or ten years ago, I'd wager he'd be the frontrunner for Harry. Lots of actors do quippy comedy well, and can stand up to all the green-screening that would be going on. But the little something extra it takes to make someone Harry isn't necessarily there in either case, in my opinion.
And Shecky, I have to say that I favor someone younger, too, and I'm your age you old coot. Storm Front Harry should be younger.
Now, I'm happy to see that some people agree Ryan Reynolds can pull it off. He's got the range, he's got the scruff, he can look way younger or older, he'd just need to debuff (*sob*) and maybe dye the hair. My FIRST CHOICE is, was, and remains, Paul Blackthorne. Since the show, he's the Harry I see and hear in my head when I read the books. Ryan gets tapped (so to speak *leer*) only if Blackthorne is unavailable.