1. I liked it because it was a challenging FPS without being too difficult (not much help for an RPG), but the "puzzles" were neat too. I enjoyed working through and really just doing the next thing that comes up. Also, finding the weapon caches were fun, but that wouldn't work as well in the RPG (as you would basically roll for that)
2. I didn't really enjoy Xen (or however you spell it). I would probably leave that out. Also, the voices of all the people were kind of annoying, but I think it would be amusing if you included them with their "catchphrases"
3. I don't really know, I think it would be difficult to capture what I enjoyed which was just running and gunning. I feel like the enjoyment of RPGs is the role-playing, which would feel weird in the Half-Life setting. Though I would be intrigued (having played the game) if you had us play soldiers trying to survive between Gordon Freeman's Rampage, the alien destruction, and the army's bombing (I mean face it, they are kind of screwed in any direction).
4. Not really sure what you're asking, nor what the style of game is going to be, so I can't really give any input. The "maze-y" confusion of the game is hard to pull off in an RPG
5. Depending on who the characters are playing, I would leave out the G-Man. I feel he is only really important to Gordon Freeman, or potentially people playing as Gordon Freeman "replacements."
All in all, I'm impressed that you're trying this, because I think it would be really tough to pull off (at least to keep in the same style of run-and-gun like the game). I did enjoy the physics puzzles of the second game, but once again, it wouldn't necessarily work as well in a pen and paper.
If you give me a better feel for what you want to do, I could give some more input...
