Personally I'd want to see Worms Armageddon and Hitman: Contracts. Worms Armageddon with Steam support would mean hundreds of players online in the lobby and lots of 8 player games (epic) while Contracts would complete my Hitman collection. Both could be possible since Worms is a Microprose game (X-Com) while all of the other Hitman games are already on Steam.

So what games that have been released for some time would you like to see on Steam?

I assume they haven't been released yet on Steam, releasing the Diablo BattleChest on there would be awesome. For a discount, of course, I just don't feel like shelling out $40 at retail. (I'd only play D2 and Lord of Destruction) If they sold it for like $25 or like Diablo II & LoD for like $20, I'd buy it on Steam.

I'd like to see EA dip into their back catalog and bring us some of their classic PC games.

None, I hope we see all older games on Good Old Games instead. They just got Duke 3D :D

Well they've had those, didn't "just" get them, but yes :)

Now that I think about it I also want to see Shogun: Total War on Steam. They have all the others, why not Shogun too.

I would rather see them on GOG, because they have no DRM, no region limits, much better support, and most important they do not overprice the games.

"I would rather see them on GOG, because they have no DRM, no region limits, much better support, and most important they do not overprice the games."

The old Fallout games plus Lucasart's back catalog of adventure games like Sam&Max: Hit the Road and Grim Fandango.

The publishers decide the prices. If they're charging more for the Steam version, that means they probably want you to buy it elsewhere.

Is KOTOR on there? It isn't that old, but I'd love to see them on here.

in Europe they (Steam not GOG) overcharge all prices

"Jazz Jackrabbit, The Sims, The Neverhood"

If you were talking about Euros, you should have mentioned that in your post. :)
However, some games do seem to have proper Euro prices, so that might be something the publisher has control over as well.

Not the place to debate it, but yes, they can control it, yet Valve doesn't openly tell them. I've spoken with certain indie devs and they were asked about the prices when the conversion was done, but they didn't explain them all it entails for the end user and actively suggested the direct conversion of $ to an equal number of Euros rather than take the value in account. There's also probably a ton of publishers or developers that didn't even bother discusssing the issue and just went with Valve's suggestion. Even Valve's own games are at most maybe 45 euros vs 49 dollars, still a big difference in value. And actually Steam is a rip off for everyone except for their excellent deals, digital distribution shouldn't be as expensive as retail, weekend deals and special offers is all they have going for them.

But I have faith in Valve, their latest deals are market experiments and so far they have concluded that offering a game for 75% off yields 15% more profit so hopefully we'll be seeing price decreases across other products once they have some more duplicated cases with similar data.

"Grim Fandango."

All of the Lucas Art classics.  The point and click adventures.. and X-Wing/Tie Fighter.

Is "All of them" too much to ask for? OK, maybe not all of them, but I want to see large portions of publishers back catalogs on steam.

"Is KOTOR on there? It isn't that old, but I'd love to see them on here."

This game's been "out" (in beta) for a while, but I think it'd be great on Steam considering its current catalog of indie titles: http://www.crayonphysics.com/

The entire "Age" series (Age of Empires, Age of Mythology) would also be an amazing pack on Steam that I'm sure a ton of people would get.

I want to see a list of old Ms-Dos games.... like kings quest and Z!

Medal of Honor series, I'm surprised that is on Steam yet.