FWIW I don't have any big interface issues on several systems, some on Arch using Gnome others on Debian running XFCE. For instance, I only have to click once on text boxes to type into them. The only time I've seen the double-click to type issue is when using KDE.
I do see a slow close (five to ten seconds after clicking the close button) on library clients, but not when using local libraries. I always assumed the slow close was the client trying to sync any changes with the server and do an orderly shutdown, etc. Network traffic takes longer than local IO for sure. That said, closing a Linux library client does seem to take longer than a Windows library client, but my windows library clients don't close instantly either (usually two or three seconds), so I suspect the core of the delay is just a part of the synchronization process.
The only part of the interface that could use a little love from my perspective is the tagging pane, where I sometimes see some input lag, but that's pretty mild. Otherwise the interface is pretty good for me on my systems.
