1) Cross platform availability is amazing. There's been a number of times when I've wished I had a game on PC that I bought on a console instead, and it's awesome to see this available now. I would have never predicted they'd offer something this nice, but they have. I'll probably purchase more PS3 games if they offer this, because it means I can play it on my choice of platform.
2) What about saves? It's nice to have it available to play for both, but if I can't continue where I left off, then this is somewhat less useful.
3) This is going to cause some issues in competitive FPSes if from cross platform play is enabled. Keyboard and mouse have been shown to general outperform controls (not to mention bots and hacks being easier to get on PC), and I wouldn't be surprised to see some outrage from console gamers.
4) And last, holy crap this is awesome I love Valve and Steam is just completely wonderful. </fanboy>
From the article:
Your game saves can be kept in the cloud to be accessed from any system.
The interesting thing here is that with the PS3, it's very easy to add a mouse and keyboard, as the console supports pretty much any class-compliant USB model. Epic Games actually tried this a few years ago with Unreal Tournament III, though I don't think it supported cross-platform multiplayer.
Portal should work well in cross-platform play since there's no direct competition: Quake III (IIRC) PC players absolutely dominated Dreamcast players. Collaboration removes the importance of this competitive edge.
In an even more amazing world, Steam is a protocol, not a service, and anyone can run their own digital game distribution service with all the convenience of Steam. Alas, I can only hope. :P
Now that they can switch to the PC, install steam, and improve their own game without having to buy another copy, valve has given them a serious incentive to give steam a chance.
The only problem is that, based on what I've seen so far, the portal 2 multiplayer doesn't seem that competitive or that dependent on twitchy control responses. If Valve were to do this with the next Call of Duty (a difficult deal to negotiate to say the least) or perhaps a Counterstrike sequel, they'd probably get a lot more switchers.
I'm going to be migrating to a Macbook once they update the processors and one of the things I'm most excited about is checking out the Steam platform on Mac.
If Valve can fill what some see as a gap in the Playstation online experience, that would be quite impressive.
For people who game primarily on consoles, you might not know what it means to have Steamworks integration. It varies depending on the implementation, it's sort of a la carte, and the PSN itself delivers some features by default. But you wont be paying for DLC on Valve games, ever. That is not a thing that will happen. And since Microsoft won't allow you to charge for something that is free on another system (recall R6: Vegas, or Burnout Paradise) it's an end run that's given Valve a lot of leverage. To sum up, Portal 2 isn't the surprise - Steamworks is the surprise. The porous nature of the PSN's functionality is an amazing opportunity for a company whose platform practically defines "platforms." Plus, there's no license associated with its use. Think about what this means for other developers, up to and including first parties. Why Sony would cede this kind of power to a third party I couldn't say, and why they would undermine the Playstation Store as a venue for premium content I couldn't tell you. This is important; we should be paying attention.
There are real technical difficulties in enabling cross-platform play.
Specifically, for skill based multiplayer games (such as most FPSes) the advantage of a keyboard and mouse over a controller is hugely significant. To a degree to where it might as well be cheating. The inevitable result of cross platform play becoming available between PCs and consoles in games like Black Ops, Halo: Reach, etc. would be that gamers quickly segregated themselves by platform, to keep games interesting.
This would mean you can continue your game while travelling.
Microsoft is in a position to do this sort of thing easily with Live, but they don't, and they didn't.
What sort of thing? Cross platform play? They've supported that (but for almost every type of game it sucks) and also giving the free extra copy is something Valve do, it doesn't really have anything to do with Sony...
Valve made a DLC for Left4Dead2, released for 360 and Mac/PC. Valve released for free on PC, but MSFT told them they must charge for it on 360, $7.99 (or thereabouts). In response, Valve had a sale on Mac/PC where all of L4D2 was $6.99, and then you get the new DLC for free.
Me, I'm excited for this increases the chances of me getting one of my friends to actually play a multiplayer game in the same room. Hurrah!
The real difference now is that they have had experience with developing for the 360 and it doesn't suit they're style. Many of Valve's most recent games have been of the multiplayer variety, and they have a strong ethic of continuing to tweak their multiplayer games over time, adding content, tweaking game balance and game mechanics, etc. This style of development, which is familiar to anyone who's played PC multiplayer games over the last 2 or 3 decades, is at odds with the way Microsoft handles game releases. There is a long QA pipeline to go through (which can be onerous if one also has their own QA pipeline) and there is a general standing policy of charging money for major content releases. And this has caused an immense amount of frustration for Valve.
The divergence of TF2, for example, between the console version and the PC version has been extreme. To the degree where it's probably fairer to call the PC version perhaps something like TF2.5, because there has been so much ongoing development in the game.
The PS3 is recognized as being the weakest platform (in terms of developer interest, game sales, profits, etc.) in the current generation, so Valve was able to negotiate a sweet deal with Sony in allowing them to put Steam on the PS3. This is no small thing, and fully explains why they have gone back to the PS3 despite the problems they've had with the console. The ability to have their own way in terms of content updates is enormous for Valve.
tl;dr Developing for the PS3 may be more technically frustrating but Sony's huge concessions mean that Valve gains the freedom to deliver game updates on their own terms which more than makes up for the technical difficulties.