It isn't the fact that you can use an external HDD or SD cards, it's the fact that you have to buy something like that on top of the system itself. The games are going to be $50-60, most likely, and who knows how much another GamePad is going to cost. Having to shell out another $50-100 for an external HDD is not something a lot of people are going to like. Don't forget that Nintendo seems to be going in the direction of offering all of their titles as downloads, too, so digital distribution is something they want as part of the Wii U, and a measly 8GB of internal storage is not going to cut it.
All these prices are purely guesswork, since Nintendo hasn't released any prices yet...
System - $300
2 games - $50-60 each
Wii U GamePad - $100-150
That's at least $500 right there, with an upwards total of $570. Add another $50-100 for an external HDD and people might start getting mad.