I might add that in all humble indie bundles linux users have paid the highest. To be fair, I don't think linux right now is a prime target for games but I do believe that it will be.
Linux will be prime time if Valve releases their long rumored Linux console box. Until then the year of Linux will always be "next year" just like always.
Another issue that Loki contended with was the fact that most of their games ran at a lower framerate than the Windows counterpart. Most serious gaming fans want to use their hardware to its maximum potential especially the FPS multi-player twitch gamers. Valve has a lot more leverage in this regard than Loki ever did. We all know (because they said so) they are working with the major GPU players to get better drivers made. But also, they don't just port games, they make them. The Source engine is very popular and they have already tweaked it significantly so not only is it competitive with how it runs on Windows but it actually surpasses it in maximum frames per second.
Another thing Loki dealt with was Linux users were a bit more idealistic in the 90's. We still held out hope that AAA game titles would just "appear" on Linux. Fast forward to the present day and I doubt many actually believe this. The problem back then was that if you expected Free top tier games to just appear you had less incentive to actually buy. I think Linux users are a bit more pragmatic now and are much more willing to pay for a proprietary binary as it's pretty clear that that's the only way you're going to play. So this is a psychological barrier Valve doesn't have to overcome anymore.
Basically, I think Valve is approaching this with the same methodical precision that Tesla is approaching the electric car space. Look at the problems that exist in an objective way and devise solutions one by one until you achieve success. Valve may still falter but it won't be for ignoring any lessons Loki could have provided.
Even though I prefer Windows (use & dev), I welcome cross-platform compatibility - more competition, more likely to be usable on future platforms, not locked out myself when I switch platforms, etc.
(Especially when you consider the recent gaben statements about win8 being a "catastrophe", and the windows app store stepping on steam's toes)
I think Carmack's response misses the mark. Valve ain't targeting Linux as just yet-another-platform.
It tomorrow MS and Apple "force" people to pay using their App store, they need some alternative.
MS was a great competitor destroyer. You will compete with them, for example with compilers, like Borland or Lotus or Wordperfect or Netscape and they will destroy you first going against your income generators and they will care about the product later, when there is a monopoly.
MS could destroy steam just using their OS control to make steam perform badly, they did it in the past with companies like DR-DOS, with media encoders-decoders(making them use code that they slowed down on purpose so their badly programmed software looked better) and so on.