My thoughts on this, in no particular order.
1) It looks good. I love that there's still active development of new mining technology.
2) Be sure to test for stability when doing odd things like repeatedly starting and stopping, or loading and exiting the program. I'm currently using another GUI-based miner, and each time I stop mining, it causes a crash in the GPU driver.
3) Consider adding support for CPUs and nVidia cards. Yes, I know they're far less efficient than AMD hardware. Still, people do have them and use them.
4) I love that you're integrating ASIC support in there. It's not just BFL, but also ASICMINER and Avalon-based ASICs that are entering the market now. More will probably show up sooner or later. I think if you can integrate support for those three platforms that are actually available now (or about to be), that should give you a solid foothold for the future.
5) Add support for solo mining. I consider solo mining to be a bit like buying a lottery ticket. It's a (very) low chance of getting a big payoff, for a relatively small cost. (The debate of whether solo mining is a good idea is never-ending and doesn't belong in this thread. I'm just saying, give users of your program the option.)
6) Will WinMiner be tied exclusively to the Dominatrix mining engine? Or will you support configurable miners?
7) Future consideration: consider adding remote stats viewing and configuration by integrating a small Web server into your app, or supporting some kind of external monitoring API.
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) Future consideration: consider allowing multiple miners on standalone but networked PCs to work together and effectively form a virtual pool, shared just by the PCs that you allow to join it.
9) Wishful thinking: Port it to Linux and make it work on a Raspberry Pi.