Feedback #1: In complex objects that have text, allow different text renditions for parts of the text.
For example, in the tree and datagrid controls, sometimes I need to show some entries in a different color, or in bold, or italicized, and so on. If there is a way to do that, I haven't found it. I've resorted to layering text elements over the control in the correct location and grouping them with the control.
Re: icon sizes - maybe you can show me a screenshot of what you're trying to achieve...surely 4 sizes should fit most cases? Mockups tries to make the 80% case dead-simple and SUPER-fast. If you like a million options, you're probably not going to be happy with Mockups.