The concept of the new media manager is great, the execution not so much. There are a couple of essential workflow issues.
1. Featured Image - this has been addressed in a couple of other topics, but a checkbox to set the current image to the featured image would be great. Having to go to another place in the dashboard just to do what was already a function in the previous version seems a little backward.
2. Uploading - Not sure why it was necessary to remove the media manager functions from the uploaded window. Another issue with this current workflow is that there are plugins which add options to the Edit Image page, which was previously available after the media was uploaded. Now you must click Edit, wait for the new window to load, find it there, then save and close the window to get back to what you were doing. Having to open a new window to get to any of the media options after you upload seems counterproductive when, again, it worked the right way in the previous version. The goal should be less steps and fewer new windows, not more.
3. Image URL - there are thousands of plugins and themes that require the url of an image, but in the new media manager there is no way to get the image URL anymore without going to the Media Manager, then Edit (which opens a new window), then copying it, closing the window, going back to the post or theme option or gallery setting, then pasting it into the field. Yet again, this worked fine in the previous version of the media manager. Opening the media manager overlay gave access to the image URL without having to open new windows and go to different parts of the site.