This is great news, especially for the products page, so many people take a look at the admin and say it is too difficult to use, even after trying to re-assure them that it is not the case.
As you say there is too much going on in one place, and needs to be simplified.
Here are a few things, I think, should be used for admin area.
Relocate the navigation
Instead of putting all the links in the header navigation, it'd be nice to use two column layout. The left sidebar should be used for the links e.g. Dashboard, Catalog, Sales... while on the right side (for smaller screens, only icons can be shown), contents should be displayed in tabs like for Catalog, the tabs would be Categories, Manufacturers, Products, Attributes...
This way, it'll seem like properly organized and store owner at any point of time will know his current location.
Add a Getting Started Page
On the dashboard, show a panel with title "Get Started With NopCommerce" and in it show some items (along with a button next to each item) e.g.
* Add few products [ADD NOW]
Setup a few products to your store
* Add new page [ADD NOW]
Create new pages like About Us, Contact Us etc.
* Configure Payment Methods [CONFIGURE NOW]
See what payment methods you'll be accepting
Show notification counts along with appropriate tabs
Some notification counts should be shown e.g. Order tab will show the number of pending orders, Customer tab will show number of customers registered but not activated etc. They can also be given at the top links where Logout and similar navigation links are shown.
Highlight the Order Status on order listing page
The order status is one of the things that caches the eye of store owner. Why not highlight the status in some way to easily find a specific order. Better user experience can be provided by using ajax based filters for quickly loading orders.
Widget locations for admin areas
I believe this'd be an awesome way for Plugin Vendors to create widgets and plugins for administration as well.
The customer lists should provide quick buttons like Send Activation Mail, Edit, Delete etc.
There should be separate page where all reports can be shown. They may include Best Selling Reports, Customer Satisfaction Reports, Best Reviews Report etc. This'll be useful for vendors as well.
The editors wherever used should show appropriate responses like tooltips, highlights on focus, help text etc.
Native Bulk Import Facility
Store owners should be able to quickly create Excel files for creating product listing and admin should provide native facility to have bulk upload. (Andrei, I did write a windows app for bulk data import which is being used by us for data entry and import to nopcommerce database. We use a fix Excel templates to import the data. I can share the code with you for this function).
Move Widgets to Configuration
A minor thing, but I think currently Widgets link is placed in Content menu, which is a bit wrong. Widgets basically display some programs and they should actually be controlled from Configuration section rather than Content.
Quick Product Editor
This one should do what it says. Show the customer with minimum required fields to create a product. This will be a quick starter for him.
Also the entire administration can be created using some ajax based system like PJAX or framework like AngularJS for fast responses to the owner.
Overall, the emphasis should be 'to show' customer minimum doors to choose the one he wants to rather than showing him with too many and confusing him where to start.
And yes, more the icons, merrier the store owner. :)
I'll get back with some more if I find.
One thing I think that's must of any user interface is that user should know his current position and any operation performed on any page, should redirect him to proper target page.
e.g. Right now, in case of multi-store website, when I am changing settings that belong to any particular tab (e.g. Localization Settings) and I change the store, the general perception is that I would want to change the Localization settings of another store, but current interfaces takes us Store Information tab.