As someone who had the original Xbox, I never played and enjoyed Halo. Mainly, because I still struggled with controlling dual sticks at one time and console shooters in general (still have flashbacks of losing horribly to Goldeneye on the neighbor's N64). As I gotten older and tried more console shooters (and dual sticks became more of a common thing in all games), I gotten better and wanted to visit the franchise that always got away... Halo. Years ago, I was use to twitch based shooter, Unreal Tournament:Gold Edition on my dad's Compaq PC, and not the slower pace of Halo. I always appreciated the lore behind the universe and even own some Halo items in my office.
I had originally planned to start buying the old Halo games and play them on the 360 this summer. There were several times where I lost ebay auctions, missed out on sales or just walked out of GS before I could get any of the older games.
After 343i made the announcement about this collection, I was elated. Now, I can play the main entries off one disc and could cherry pick moments that I want to play. I'm still missing out on ODST and Reach (which is a bummer) but at least I get to play through the main story beats.
Overall, I am over the moon about TMCC comibg out and so are people that are in my unique situation. Is it a system seller? I don't really know but then again, I usually don't believe in only one game selling a console (well, maybe Smash bros or MK8).