I'm not sure about the whole series being planned as a trilogy to begin with though. From what I remember, the first game had a pretty clear cut ending. From what I remember, it was pretty much:
-Covenant attack humans
-humans follow Covenant to some ring thing (Halo) and crash
-Covenant want to activate the Halo as a weapon
-The Flood
-the end
After the end of Halo 1...
...Yeah, after that end of Halo 1, after the game feels like it's over b/c Halo was blown up, the very short conversation on the ship b/t Cortanna and Master Chief. It hints that there is more to come.
You're right -- most of it was tied up and all, but they STILL hinted at the last few moments that there might be more to come. And again, in terms of completion, that's still a no-no. A small no-no, but still a no-no.
Halo 1's ending was nowhere as disastrous as say R6: Vegas One's ending or say Assassin's Creed's ending.
The Flood, the backtracking, and the cut-and-paste levels (i.e. The Monitor's levels with look-alike floorts) felt disastrous to me.
Won't speak on H2's ending until I get a Windows Vista or above and actually play it all the way through -- it's only fair. I heard it's a cliffhanger and a half, but yeah -- I'll hold off on my opinion on it.
I think its just alot of us underwhelm the multiplayer gaming audience, which is much more massive than any of us can comprehend. Its the same reason that Counter Strike exploded. Halos success has really nothing to do with its single player campaigns at all, and its all multiplayer frenzy, and those that enjoy multiplayer FPS gaming love it, Im not into it so I cant say much of anything, and neither can alot us because the only multiplayer games we are really into are ones we can just play with each other and WoW. Its really all it is, I just hate when we link the success with its story and campaign, which has its faults but even thats not terrible, it gets blown out of proportion.
I think Halo 1's SP first half was great filled with lots of variety and everything, while the 2nd half was....a disaster in comparison -- it was as if they decided to stretch what should've been a great short game out into say a on-and-off doubly-as-long experience. Halo 1's MP was rock solid on the PC, but it was really nothing we really ain't seen on the PC before, honestly.
What really made Halo on the consoles was...they really had no other FPS like it around. At that time, name me another great FPS on the console that wasn't first on the consoles. Halo's gameplay was purely designed around the console gamepad. Finally, that part was done right for a FPS -- might not feel as good as KB/mouse would for a FPS, but this was as close as you could get it (Without using KB/mouse). Also, there was no multiplayer experience like that on the consoles really before Halo. On the PC, sure -- we already had plenty of MP experience with these type of FPS games, some 10 years before-hand; see Doom series, Duke 3D, and Quake series, Unreal series -- and that's just for starters.
But, really -- RTCW PC should've never got a 9.2 from GameSpot. The MP was fantastic, sure -- but the SP was just solid and good. Honestly, MP and SP components should be scored separately, anyways -- b/c when people look at that 9.2, they're thinking of the whole picture, that both the SP and MP are both great. In Halo 2 console version's case and RTCW PC's case, it looks like just the MP was scored accordingly if you read what the hell is written there. Hardcore gamers here like us all on OWNet, we know better -- we read the reviews b/c the score sometimes is a real head-scratcher and doesn't reflect the score that is given.