I too would have to side with Halo being one of the all-time best FPS shooters (regardless of platform)
I will try to elaborate what I liked about when I initially played it. Please bear in mind I too prefer a keyboard and mouse and have been a PC FPS player since Wolf3D...I will also go ahead and note that all of the following statements are merely my opinions (some educated, some preference).
Firstly, when I first saw Halo, it was using graphical effects that were scarcely implemented well even on PC. Bump-mapping was in effect and looked great. Particle effects and overall texture variety was high and gave Halo a different and polished look. The overall presentation of Halo was fantastic and still looks good to this day IMO. Also, the game kept an incredibly stable framerate regardless of the absurd amount of enemies on the screen.
Secondly, the story in Halo I found highly engaging, well-written and of a generally higher level than most video games (and better than most Sci-Fi material of other genres too). Taken in and of itself, regardless of the game's graphics and gameplay mechanics would have made the game enjoyable.
Thirdly, gameplay mechanics were and are still top tier. Certainly, Halo was not the first FPS to implement melee combat, but it was the first to do so and have it feel seamless and natural to use (ie: not a gimmick). In fact, that one element of gameplay is one of the reasons multiplayer is so enjoyable in Halo. Melee has only since been implemented as well in Dark Messiah and Oblivion to date. Aside from melee, the limitation of two weapon types gave the game a hint of realism and strategy that was not really present in other FPSs of the time (notice FEAR used this kind of intentional limitation as well). Grenade usage was also well implemented. Previous FPS titles had you switch to Grenades to use them, and doing so generally broke the pace of the gameplay. In Halo, throwing a grenade is so natural that new strategies become relevant. Vehicular combat is still one of the shining points in Halo. All the vehicles were a blast to play with and offered some exciting strategies themselves. Last on this point is that the game is incredibly difficult on Legendary (especially playing Co-op). However, the difficulty was never unrealistic, in fact it only added to the feel of the realism in the gameplay. Enemies were not super-powered, just responded ever more like intelligent opponents.
Fourth, AI was absolutely unparalleled in Halo at the time. It has only recently been supplanted by FEAR (and perhaps some of the latter Rainbow Six titles though I can't say from personal experience). The Covenant acted as cohesive units and would outflank, send sorties (and point men) and surround you. The flood just overwhelmed you with sheer number and strength (as they should be given that they are mindless drones). Given the two enemy archetypes, Halo was the first FPS to blend strategic combat versus sheer adrenaline-pumping run and gun action.
Fifth, multiplayer, multiplayer, multiplayer. I really can't say what it is about Halo multiplayer that makes it so addictive and fun... Perhaps it has to do with the fact that you can see your opponents screen. This adds a different element to the combat, being able to know what your opponents are doing. Strategies change and become more involved as a result. Secondly, your controller limitation causes some degree of specialization in different weapons (a high sensitivity is not conducive to sniping, and low sensitivity makes you too slow in turning to effective battle close range. I'm not sure what they screwed up on the PC port, but they didn't quite get the response of the controls right. The maps were varied and well planned and lent itself well to the fast paced duels. Of course the addition of sitting with your buds, drinking some brews and talking smack always helps push the fun factor. Teamspeak will never be able to deliver that kinda of mood (of course not being to turn around and punch one of your friends might be one of the reasons why
. Of course I can not neglect to mention that Co-op mode made the single player game immensely enjoyable.
I've seen plenty of criticism for Halo, but really not until after the shoddy PC port was released and PC-exclusive FPSr's started playing it. I can understand some of the criticisms. I myself had a hard time playing Halo 2 after playing some of the newer PC FPSs because of the controller. Some of the other criticisms I've seen were based on the level design. Some found it repetitive... which is really the case with all FPS in certain areas. What makes a game still enjoyable to play is the overall gameplay and combat was exciting enough in Halo to make it fun to play over and over.
Apologies for the over-long post, but initially the post asked for specifics, so there you go.
Just as a follow-up, I still haven't beat Halo 2 (the controller got to me too). I'm hoping the PC port is better this time around.
Halo for 
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