I also agree with the opinion given here by others.
We must need to focus on improving UI for the front-end theme as well as admin side. But without loosing the simplicity, I think minor modification and fixes that improves UI to look it more modern would be good enough. A default theme should be simple but attractive. Moreover, the bugs that johndillon reported related to admin UI are really something that causes admin / data - entry users to loose lot of time. This should be taken care while development.
I also believe nopCommerce should revamp its existing website - because when potential customer (end user) sees it, it looks like an old website.. especially when all other competitor carts are looking very modern and light. For example look at this.
Demo store sample products should be changed to reflect the newer products. The older products gives a wrong impression!
I also recommend to have a better features page with more details on each features, may be along with picture would greatly help in convincing more customer to try nopCommerce.
Showcase page on nopCommerce website needs an improvement as well. If we have more than few entries then its better to organize it per industry. May be we could have rating system on sites submitted on the showcase, and we display only the best rated few sites on default page. Moreover, the page design for the showcase page needs to be presented in better way.
Additionally, nopCommerce website should be now mobile / tabs friendly.