Guys, don't make me split this thread or start deleting posts. I really don't like doing that. Bad things happen. Stuff. You know.
Kaz, your "HL's engine sucked monkey nads" comment was wonderfully descriptive, especially coming from someone who is always reminding us to back up our claims in the various debates we've had.

Mind you, you did explain a bit more a few posts further on, but still...
TC: Back when I had the inkling to do a Freespace FPS, I decided to do it in the Serious Sam engine, for a few reasons:
1 - It handles non-standard character movement scriptings very well
2 - It supports multiple gravity fields (good for hallfight scenes)
3 - It's able to scale down graphically to support very low end computers (think 3D graphics at 320x240 resolution) or upwards to support bump mapping, reflections, haze, fog, colored lights, etc.
4 - IIRC, it comes with all the MODding tools you need AFAIK
5 - It can probably be found for cheap in bargain bins, for those who don't have the game.
Anyway, the character model animation flexibility and multiple grav fields were what really made the decision for me.
Unfortunately, not many people around here knew how to or were willing to earn how to help me MOD, plus army got in the way, soo.... it never was conceived, really.