I never really got why Halo was so popular. I'll probably TRY and rent this.
Well here's what it is for *me*.
Story. Admittedly, you need to read the books to get the best of it, but I really liked the Halo story. None of it is totally original, but it all hangs together well. The in game cut scenes and dialogue are well done also.
It was pretty much the first console FPS that I found playable. The control scheme, whilst dumbed down from PC natch, felt good.
The weapon mix and vehicles. The decision to only allow you to carry 2 weapons at any one time was looked on as a risk at the time, but it worked well, forcing you to choose tactically what to carry.
The vehicle battles were great. The different styles of the Warthog, Ghost, Banshee and Scorpion made for some great tussles.
AI and character speech. The AI, especially on the higher levels, was pretty decent. And they recorded a massive amount of chatter, so you rarely got the same marine banter twice.
Music. It's somewhat derivative (what isn't in scores these days?) but it's atmospheric and works well.
That's it for me. It's by no means perfect (Library is laziness, though I don't hate it as much as some) but it all just fits together well.