For what it's worth, I think the new admin is a vast improvement over the old admin. But, some key basic features don't work well yet:
1) Product images and image handling: should have auto thumbnail, and dynamically allow for many images. On catalogue side, it's would be much better not to open new window to show larger image, it should work similar to ask.com (binocular) or use flash .... better and smoother than opening new window. Small thumbnails with binocular action on shopping cart, invoices, and shipping slips would be great.
2) POS functionality (for orders that are called in). Ideal may be to able to assign certain the catalogue side users as POS cashiers who are allowed to change customer on the order. Professional and customizable invoices and shipping slips (pdf would be great).
Also, I was wondering if the db structure has changed in v3? Can we use the new admin with old store?
I know, I know, I know all these are available in one form or another by various Contribs, but it's all a real pain to get working well since osC didn't/doesn't have a practical/usable plugin architecture.