I'm sure the 3DS will be like every other handheld I've ever had: Some feature(s) will be annoying to me in some fashion, but given time I'll get over it.
First handheld was a Gameboy Pocket. It was nice, but it wasn't backlit, but I got an accessory that took care of it.
Second handheld was a Gameboy Color. It was a bit bigger than the Gameboy Pocket, which made it feel awkward until I got used to it. Also, not backlight, but I got over it with an accessory.
Then came the Gameboy Advance. My first thought was "Did they manufacture this sideways?" Got used to the ENTIRELY new feel of the thing pretty quickly. Still no backlight, so I had to get an entirely NEW accessory for that purpose. Although I did like the battery indicator light. Also wasn't a big fan of the smaller game carts.
The Nintendo DS pissed me off the most. Internal batteries (I like the idea of me being able to change batteries on my own, so that way if it dies on a long trip I can just replace it and charge them up on my own convenience). Little backwards compatibility (no Gameboy/Gameboy Color support). The new "game cards" I didn't care for at the time, since I feared they'd be so much easier to lose. I also for some reason didn't care much for the touch screen, I thought it was purely gimmicky (even still, I prefer to use the buttons and only use the touch screen when forced). In the end, got over a lot of those annoyances and now the DS is my favorite handheld.