In short, Microsoft is dropping support for XBox Live on the original XBox, which among other things means you wont be able to play original XBox games (including Halo 2 most notably) online.

My thoughts on this are that it sucks, but is unavoidable. I'm also kind of glad I'm a PC gamer, as this doesn't really affect me anyway!

1) If you ever needed proof that dedicated servers are superior to to matchmaking, then this is probably it.

The fact of the matter is that server/multiplayer networking costs money to run, so it makes sense that Microsoft (a notoriously aggressive major corporation) will shut it down when it stops making money. As Microsoft controls the networking, then even if other people want to pay the money instead, they can't.
With server files and dedicated severs though, as long as there are people wanting to play the game, it will remain available.

How I feel about this is pretty simple; I want the option to go back and play any game I have paid money for, as long as its reasonable to do so (i.e. there are other people playing the game). In some cases (Quake, original Unreal Tournament), the only way you can experience certain kinds of games is to go back and play the 'oldies', as their design sensibilities are out of vogue at the moment.

2) Are these games even 'Old', anyway?

Halo 2 was released in 2004, a mere 6 years ago, so to call it 'old' does seem pretty odd. As a point of reference, Unreal Tournament 2K4 was released that year, and Battlefield 2 the year after, and both these games still have fairly sizable payer bases, BF2 especially. For this reason, I am a little disappointing that with devoted player bases of games like Halo 2 will simply cease to be, as I'm sure they have many years of enjoyment still in them.

3) You shut down MY game!??

Although the games studios of the original Xbox games signed a deal with Microsoft, you can't help but feel for them, as their hard work is essentially being trashed right in front of them, and there is nothing they can do about it. Whereas PC game studios have the option to support their games if they value them that much (Battlefield 2 received a major patch just a few months ago, in fact), Xbox studios have to stand and watch as years worth of balancing, bug fixing, and community building is flushed down the toilet. Some companies really pride themselves on post release support, so it must suck to be them right now.

At the end of the day, though, I feel this is just one of the many aspects of console gaming you just have to accept. If you want convenience, you have to let other people do the work for you, which which means ultimately they decide what you can and can't do. Though I do feel sorry for the gamers and studios that will lose something because of this, they did have a great 9 year run, so it's not all bad.