I'm not sure this would be an enhancement, because the current behavior is to download all games at once as the internet connection permits, this would require adding a weighting mechanism to divert bandwidth to higher-priority games while keeping the total throughput at maximum (as a spill-over of left-over bandwidth from one game to the next).

If you want to play a single game as soon as possible, then tell it to run and the client focuses on that one game. The launcher does not need to stay in the foreground. Also, by default steam suspends the download of games not being played so that the user gets the best online gameplay experience. In Windows, you can alt-tab out of the game and tell steam to continue downloading, but that is risky (high probability the game will not recover) in linux.

Is the specific use-case where a user would want to hop immediately from one game to the next as they become available? is the use-case where the user is on dial-up or another severely limited connection?

Edit: Or are you asking for a button that gets the game cache, but does not install it to the user account?

@Tele42: What you have described is the behaviour of .ncf (which is being phased out) and .gcf games. Games in the newer .acf format (i.e. almost everything except Valve games and a handful which haven't been converted yet) only ever download one at a time and do NOT automatically pause when launching a game (as a side note - if you want to alt+tab out of a game in Linux and can't because the game has placed a grab on your keyboard, try using shift+tab to bring up the overlay which releases the grab allowing alt+tab to work. Alternatively you may set the game to run in a window).

With .acf games, clicking download sets it as the active download and pauses any other .acf games being downloaded (they are marked as suspended in the interface). I assume the OP wants a way to add a game to the download queue without setting it as the active download (it's possible to set a different game as the current one being downloaded it by "pausing" and resuming it, however AFAIK there is no way to tell Steam which game to download NEXT when downloading > 2 .acf games).

@DarkStarSword: Thank you for the clarification. Would another acceptable solution be to re-implement parallel downloading and I wonder if this issue is cross-platform ... I will test shortly for my own information.

Edit: Yes, this issue is cross-platform.

re-implementing parallel downloading still wouldn't solve the issue due to various game sizes. If I wanted to download, say... Shank, TF2, and Offspring Fling (in that order), a queue would work but parallel downloads wouldn't because Offspring Fling would download first.

Now that some sense has been knocked into my head, this report appears to be related, but not the same as #540

#540 is essentially a bug report whereas this is a feature request.