The user interface is 'way different, and the .ses file format has changed, which means that you can't open .ses files recorded in 1.5 in 3.0, or export your 3.0 .ses files to another computer using 1.5 or earlier versions. You CAN open earlier .ses files in 3.0, but it will do a conversion to the new format (you can still retain the originals and save the converted ones that you want to use in 3.0).
ASIO, VSTi, better synch to video (I understand -- I don't use that feature), takes up more space on your hard drive (just like everything else these days), unlimited tracks....
For me the I/O configuration and the method of saving default settings drove me crazy after using CEP 1.2a through AA 1.5, so, for now, I'm back to 1.5.
There are changes to the mixdown protocol, the way it saves (you have to save first, before recording; and it saves HUGE amounts of data so that your drive fills up quickly), the reverb and compression presets, and other features. For me, it was like starting over with just enough similarity to the old program that I got confused. I'm sure that Adobe was eager to get their hands on the software and make it "right" by their standards, but I liked CEP fine.
Part of the difficulty is that my computer is having some problems with the boot drive, and I have to reload the ASIO drivers every time I want to use the later version. When I replace it I'll likely give 3.0 a whirl.