The goals for 4.x include everything we don't get done in 3.x before the release (the release is based on having a working version ready when Drupal 7 is ready, not necessarily a fixed feature set.). Specifically things which break api's and or Ubercart contributed modules. We have a goal of not breaking the functionality of contributed modules between major releases. The major change that comes to mind is decoupling products from nodes which is to say making a product a field (set) on a node.
I don't exactly know how to address your "More or less simple shop or wide e-commerce platform?" question. I guess the answer is that we want to make Ubercart more modular in order to have more features and be more of a "wide e-commerce platform" while using install profiles and features ( the module) to create an easier install and default setup for those simpler shops. Does that address your question? I would be interested in hearing an expanded explanation of what you mean.
Agreed, we could do better with the issue queue, but we are pretty focused right now on getting the 3.x release ready. We would love some help creating and testing the patches that are out there.
I don't know that I agree Drupal 7 is a long way off. I expect the next release or so to be a beta and for there to be a production release in the next few months. Our plan is to go beta and release on the same schedule as Drupal. That being said, Any patches for 3.x will be considered (read welcomed) before beta. After beta it may be postponed if it breaks dependent modules or seems too big to test and work out in the beta process. If it is postponed, it would still be welcome, but just pushed to the next release.