Thinking that with a really optimized, organized and structured code you'll make your software the newest killer app ever? Been fast and stable is just a couple of the requisites users look for while choosing their software. Among all the requirements that users instinctively looks for in a software, usability is the most decisive of them.
A software with poor usability makes user feel angry and disappointed about the software. Imagine that someone is trying to use your software in a low profile machine, and he starts missing some clicks because a process he is trying to do is very confusing and has many steps based on clicks and keystrokes to be accomplished... Gather this with the fact that he is in a hurry and his boss is mad about the time he spends to do something with the software. I don't believe this person will ever love your software, nor he is going to indicate it to his friends, even if the underlying code of this software is well structured, organized and optimized. (This kind of software is not recommended for persons with cardiac problems)
We have a lot of effort trying to spread software development techniques all around, but few trying to spread usability, accessibility and internationalization techniques. Keep in mind that your software is used by human beings, and usability really matters.
Spend some time reading some Human Interface Guidelines, choose one and stick with it. Also, make interface test suites and watch how your users work with your software. Try to count how much time they spend doing some tasks, and how can you decrease the time spent on each task. Try to make your interface as clean as possible, and keep in mind the KISS concept.
Some of these advices I didn't have time (yet) to make on Umit, like interface test suites and watch how users are working with Umit. But I had some experience on interface test suites at my job and realized how important they are for software development and software usability improvement.
Although I'm not an expert on usability (yet), trust me: your software is not about code, but people and their desires...