The main area of the UI is the tabs; this is where you use all of Soulseek's features, and where your vision is focussed. The toolbar at the top contains context-sensitive options for individual tabs and even sub-tabs.
1) Toolbars are static 99% of the time. Having it context-sensitive is not what a user would expect, and it's easy to forget.
2) Tabs have a confusing mixture of active and inactive elements. e.g. a) the options tab, where all elements in the UI tab can be changed, but in the login tab only one can, all other options look like they can be edited but can only be edited from the toolbar; and b) the search tab, where you search and filter under the tab, but change sort order in the toolbar. Because some elements are in the tab where you expect them, you again don't look elsewhere for controls.
3) The toolbar is visually a long way from the active area under the tab. It's easy to miss completely, and this is even worse as there's a lot of empty grey space between e.g. the search results control and the toolbar.
Controls should be near what they affect, having the appropriate buttons moved off of the toolbar and into the main area in their appropriate tabs would make them more obvious.