A brilliant game, and sure to be remembered as the divine bridge linking forebearer to promised land.
First and foremost, Halo 2 is what I would call a near perfect game, and despite even Bungie's claims of not being able to perfect it, it is still fantastic. Now I would completely force you to play Halo: Combat Evolved first, but it's your choice. If you go straight for the second, be prepared to be confused by the story, as it is heavily prevalent in the Halo series. The storyline is very well contructed, with very good writing involved. The war with the covenant has been brought to Earth, but now you not only have the human perspective, but the point of view of the Covenant as well. I'll leave it at that. Now, to address many complaints about the cliffhanger ending, it is a very common element of plot. There is a cliffhanger here, and the reason why most people are so enraged by it is because the story is that good, and they don't want it to end. This is a very clever and somewhat cruel tactic, as it creates excess desire for the next part of the story. To me, I don't mind the cliffhanger. I enjoy anticipation greatly, and this is no exception.
The core gameplay is still the best of any console. The controls just work. Everything flows seamlessly and the scheme is very comfortable. Mindless shooting is not present here, as killing is satisfying, and you will enjoy every moment of it. The single player is a good length, it is not short if you actually take your time to enjoy it. People who wish to rush through things will be disappointed. The campaign is extremely well done, enhancing the already strong story brought on from the first game. In addition to that, you will find yourself playing through it many times over, both by yourself and cooperatively. Halo 2's multiplayer experience remains unmatched on Xbox Live, offering options the world (or universe rather) over. The online multiplayer is brilliant, and the only things you might find annoying about it are the actual people playing with you. Don't be surprised if you find yourself still engrossed in the multiplayer component of Halo 2 for several months or even years for that matter.
On to the technicalities. In 2004, the graphics of Halo 2 were unmatched. The visual style is incredibly sharp: with realistic textures, fantastic lighting and effects, beautiful crisp explosions, and a solid frame rate, it's easy to be immersed in the design and effort put into the game. Sound effects are some of the best I've heard, the voice acting top-notch, and the orchestrated soundtrack so well composed that you will probably find the need to buy it.
To conclude this, Halo 2 was the best game to come out in 2004, and one that will be remembered for years to come, along with the first and third in the series. I recommend it ridiculously highly.