As a long term Bungie fan (ever since Pathwys into Darkness) I will be buying PC Halo simply because it's Halo. It really bugs me that after all this time I still haven't played it - and I was download shaky-cam footage of the Macworld Expo the very day it was announced! I need closure! Never got around to trying it on an X-Box because I knew those lousy controllers would spoil the experience for me (who in their right mind designs an input device which relies primarily on the THUMBS to control it?!?)
The aspect of Halo which still looks strong is its AI - I've heard heaps of good things about enemy behaviour in this game. Also the physics engine, true to Bungie's history (Think 'Myth' and 'Marathon') seems geared primarily towards good silly fun.
Editing tools? Yippee! I wanna see old Marathon maps ported to Halo
Lack of co-op will be a serious blow, but won't kill the game for me. I'm still hoping this will be a good adition to my LAN parties, I have many friends who organise X-Box parties specifically for Halo, and these are people who HAVE decent PCs already but just would rather experience Halo.
But with so many other hot titles scheduled for that time of year, it will surprise me if Halo gets a strong online presence - not with everyone playing HL2.