For me it's about ease of jumping in
1. If they would only package the Steam client for 64-bit Linux! It's really off-putting to have to install 500 i686 libraries. I have not installed Steam since a re-install simply because of this reason. You mention 64bit client to a Windows person and they start a 50 page thread on the benefits of 64-bit addressing vs 32-bit addressing and how it's not worth it for Steam, but they totally miss the issue.
2. gfx drivers, as already stated, are an issue. I use AMD HD7750, and RadeonSI is perfectly functional in the desktop, but underperforms Catalyst. However, it's such a hassle with AMD binary drivers, having to wait for compatibilty with X11, having to reinstall on every kernel update. I'd tolerate lower performance just for the benefit of full compatibility between RadeonSI and the rest of the Linux stack. If I had an nVidia card, then the binary is the only alternative, and works very smoothly, it's just that im a free software enthusiast.