@AC - what has changed
Form factor has been made smaller
Power requirements have been reduced
Best thing? It's now $20 (before tax/delivery). I am not aware of anything remotely this powerful in this price range. Remember, decent 3D, 2D, camera ISP, 1080p30 H264 encode/decode, 1Ghz (overclocked) processor, well supported Linux distro (plus others), great community.
I think that is a pretty good deal.
And I think you are wrong about the A being more popular, I expect sales of it to fall to zero. This is so much better. The ONLY reason to still buy an A is if you already have a case for an A. And since cases cost from about $5 you may as well just buy a new one for the new form factor. TBH, if case compatibility is the only argument you can come up with, the A+ is clearly already a success!
The reasons for 1 USB is that the SoC only supports 1. To add more, you have to add a USB hub to the board which means much more expensive - basically the old model B. You want more USB? Get a B+. It's only $15 more.