Right, we need more information please :)
Could you add the output of apt-cache policy steam, how much disk free space you really have df -h would be ok, and if you have Steam set-up in a non-normal case, like using symbolic links or having installed it on an NTFS/XFS mount.
When was the last time Steam was working for you, did it apply an update and then have this issue?
The D-BUS warning can be safely ignored.

I made some space on my (ext3) home partition, deleted ~/.local/share/Steam, and apt-get purged everything steam which meant I was able to reinstall fine.

It's possible when steam was initially updating I missed the error message, and subsequently my Steam directory was somehow corrupted. I can say that when I reinstalled, I got this error message after steam had downloaded 150 MB of updates:

"Fatal Error: Steam needs to be online to update. Please confirm your network connection and try again."

This might be because my routes are static in /etc/network/interfaces . It's not the first time I've seen this error message, I might have seen it yesterday when I loaded up steam (which I installed a few weeks ago) and it said it was updating. (Maybe I had enough space when Steam downloaded the update, but by the time it was downloaded and ready to extract there wasn't enough space?) I was definitely out of space, but the Steam launcher was totally broken by the attempted update (and had at one point worked.)

Unfortunately I deleted my corrupt config folder so it can't be inspected. Probably just as well to resolve this. :)

It might be worth trying to reproduce by running the installer when you don't have quite enough space on your home partition. It's possible I even saw the "you don't have enough space" warning, but forgot about it, and was later confused by why steam just did nothing when I tried to start it, which is the main bug.