At this point I prefer developing the core and features instead of developing the GUI.
I couldn't agree more. Actually I like OS client design. It's super clean and simple with 100% usability. Any custom design should be at least as good as OS design, otherwise it'll be just meaningless gradients and nice buttons.
I was surprised why I used my first Bitcoin wallet and found it looked like software I was using a few years ago on XP. But I think some people kind of like that. Sometimes a really polished 'cool' website and design can actually put people off. Number 1 priority people want functionality and simplicity. Examples include Reddit, Drudge Report and DGEX exchange. Good design has to have synergy with the client in the end.