The thing about this is that its like a runaway snowball. The more publishers that join up means more people sign up means more people for publishers to sell games to. Which in turn makes the whole thing more enticing for new publishers and developers to join up.

We may never see all publishers using steam but a large portion of the major publishers probably will. It just makes good business sense.

Instant access to free demos, trailers, friend chat, groups, voice chat, instant messaging, mod tools (things to make game mods) and a launch pad for all my games (no changing of discs).  Along with this I can join games my friends are playing right from my friends list as long as I own them.  In the store I can check out metacritic ratings right in the shopping list.

I can also set up a media center (add my favorite videos), track my stats and playtime (viewable by my friends and people in my community groups), and directly download all games I purchase straight to my hard drive. I can even purchase games before they are released, pre-install them, and they'll be unlocked instantly at Midnight on the release day.

Along with this are huge deals (Like every Unreal Tournament game for $54).

Oh, and the whole service is free. Basically it makes PSN and Xbox Live online systems look primitive.

It is a lot of things really:

- Instant access to buy and play new games.
- Community features allow you to find out what your friends or "groups" are doing and chat/voice chat in and out of games with ease
- Demos, News, Trailers, etc...made readily available
- If you re-install or get a new computer you simply re-download the steam client and tell it which games to install from your list. Then you can walk off and when you come back all your games are ready to go without you having to watch over them.
- Some great deals, for instance if a sequel to a game comes out they will often offer a package deal where you can get all of the games in the series etc...and they occasionally do pre-order discounts of 10% on games.
-When you pre-order a game they usually let you download the encrypted version of the files and then when its officially released you can quickly unlock the files rather than having to start downloading them along with thousands of other people at the same time.
- Automatically checks for and updates your games so you're using the latest version, it will usually show you patch info when it does this.
- Stat and Achievement tracking similar to the Xbox Live service.

There is probably more but that is off the top of my head....

edit: bah, Naz beat me to it 

Ah, yeah I forgot a couple features.

You can always reinstall all your games on any new computers without any issues, and uninstall and reinstall any game you've purchased at any time.

You can also register the CD Key of a game you've purchased in a store on Steam (that's also available on Steam) and you will be able to install it on Steam as well, granting you all the features you would have if you'd bought it on Steam in the first place.

If your game wasn't available on Steam you can "add a non-steam game" to at least give you the ability to launch the game from your Steam Games menu, keeping all your shortcuts neatly organized.

Steam is simply right now the ultimate gaming service.   I can only hope other companies including console companies do their best to emulate it in the future.

The biggest reason that all gamers should LOVE steam is because they offer what is arguably the most full-featured online service bar none and they do it for FREE. This sets the standard and it makes it hard for companies like Microsoft to justify the reasons why Xbox Live and Windows Live subscribers are paying for what Steam users get for free.

Not entirely true.  Very few third-party games allow you to register a retail CD key through Steam.  It would be great if every game allowed you to though.  As much as I love Steam, I love having a box more.

Steam = best online gaming service currently IMHO. PSN is still young and free but I still struggle with Live costing anything (even if it isn't much really).

At this rate Steam is going to be the PC Online Service to go with, Games for Windows doesn't come close right now IMHO.

Hopefully they finally bring the Linux Steam they've been working on out at some point. Then I can ditch Windows for good and use something that actually works.

 XP Service Pack 3 fixs and improves a ton of crap. Your fine, just dual boot

and I thought xfire was awesome 4 years ago, just because I could know what my friends were playing, and where... 

Steam grew a lot, and the deals you get once in a while are just amazing. last week they had Two Worlds for just $10. too bad they got regional lockouts, now I can't get Bioshock "cheap" anymore...