Well, if they increase the size of the flash memory, it's a very late change as it was 8GB nearly all this last year on dev kits.
For the USB 2.0, don't know, maybe the interest of a Nintendo branded HDD is to reach a certain level of access/loading time, revolutions per minute, to run the games comfortably.
I did a quick check on this as far as (continuous) read speeds are concerned.
The bottom line is as long as you use a good quality USB2.0 drive you should be fine.
Nintendo would be smart to publish a "consumer guideline" document on what drives are supported so customers don't end up buying a cheap and slow drive and missing out, and also in order to ensure developers that software installation is a non-issue.
the details are as follows:
Blu Ray x1 read speed is 36 Mbit/sec but no commercial drive is any slower than x2 because you need at least 54 Mbit/sec to play movies. Sony went with the cheapest drive possible which is x2 (=72 Mbit/sec) because they needed to keep costs low. As a result they created a severe bottle-neck in data access (even slower than the 360 DVD drive) and this has hampered developers all through the generation. Nintendo would be stupid to repeat this mistake, especially since prices have gone down a lot since 2006.
Therefor I am assuming Nintendo will go with x4 Bluray which means 144 Mbit/sec for continuous reads. This puts Wii U slightly above the 360, which had a x12 DVD drive at 132 Mbit/sec maximum read speed.
Now I don't know what MS used for a hard drive but a good USB 2.0 drive today manages
actual transfer rates of 22-26 MByte/sec = 176-208 Mbit/sec. This is also something I can attest to having recently purchased such a drive and doing the comparisons myself. Therefor as long as the drive doesn't drop below 18 Mbyte/sec (=144 Mbit/sec) during continuous transfers, it shouldn't make a difference to the software as far as data being delivered.
There are crappy USB 2.0 drives in the market that don't do 18 Mbyte/sec continuous, but as long as customers know to avoid them, we should be fine.