Having played all of them, the very first one was flawed but fun in single player:
1. sticky grenades added notable gameplay -- and even better then enemy AI used them back at you. 2. for the time, good and varied enemy AI. 3. vehicles were a nice twist even if the steering was flawed 4. replenishing shield was novel and added gameplay 5. Melee was a nice addition that could save your ass.
So anyone who says the first one was rock bottom bad? Cannot agree with you.
That said, the PLOTS of Halo are horrifically non-nonsensical and terrible. They stopped making sense in Halo 2, and it only got worse later on.