I'll admit I'm not a specialist in PC gaming, but my understanding was there is such as thing as "graphics lag". This happens whereby too much action occurs all at the same time within a game and your hardware is unable to render the graphics, resulting in a slowdown in frames per second and thus making it difficult to see what's going on. If the person you're fighting against has a faster PC, they don't experience this slowdown, and therefore see all the action, giving them time to see where you are and shoot you (or whatever is the equivalent in the game you're playing). By the time your PC has caught up, you're dead, and you didn't even see it. A quick google showed up
this which seems to go into the problem (although obviously, being a graphics card manufacturer, I would take the importance of what they say with a pinch of salt).
I've experienced this on the original PS2 - the difference was, since it was the same hardware, it affected everyone at some point and was pot luck. With PCs of course, everyone doesn't have the same hardware, therefore giving the edge (albeit admitedly not a major edge) to those who have more powerful kit.
I'm not saying other forms of lag don't exist - I suspect internet connection and latency are far more important factors, but I'm pretty sure I'm not wrong in the fact that the above can have an effect sometimes.
If this is all bull, feel free to correct me, but it's certainly logical from what I can tell.
However, my second point about ensuring games run on all Steam machines seems like a much more likely issue for those who buy the cheaper, lower spec Steam machines, and one that would concern me.