I strongly disagree. The analog nub is fine the way it is. I remember when I first saw it, I was very happy with their choice. My mind instantly remembered the c-stick on the Gamecube controller.
There are a few problems with a concave nub:
I can see that the concave design works on an Xbox 360, I can testify the Neo Geo Pocket design is awesome and I hear the 3ds slider nub works great. But remember, this is not the Zero's main input (your thumb will usually be on the d-pad), the nub on the Zero is secondary and doesn't look too comfortably placed to start with. I feel a concave nub would only make it worse because your thumb will be at an angle, which would not be good for getting grip on such a tiny concave nub. The concave nubs on the 360/NGP/3ds works because it's where your thumb naturally rests when you're holding the controls, so it's easy to get your thumb right in there, work it around and have an enjoyable time... Hey! Stop giggling! I know what you're thinking!! STOP!
Another thing to consider: unlike first party controllers, this nub probably won't be coated with that neat, grippy rubbery stuff that I imagine NASA invented. It's going to be hard, 1980's Ninja Turtles toy quality plastic. That's probably why it has little pimples all over it -to increase grip without rubberizing it. I remember many years ago I wore out the (hard plastic) original N64 controller and I replaced it with a 3rd party controller that had a (hard plastic) concave design. Man, it sucked. It tore apart my thumb in a week.
I'm not about to say it's perfect though. For one thing, in qbertaddict's videos it worries me that it has a lot of wear right on the top of it, which I suspect is probably from rubbing away on the inside of his pocket or its case. Imagine how quickly a rubberized nub would get chewed up.
Lastly, as gibberish mentioned, it's way too late to change, the Zero should be shipping in about three weeks and I didn't hear anyone complain about the nub this whole time. If it tortures you that much, you can just mod it yourself; c'mon this is made by GCW, not Apple (BUUURN!). Saw off the top of the nub, steal a nub from a 3ds, join them with a tiny bit of JB weld then post your results with video.
I couldn't care less about the X and Y buttons, I'm not looking at them while I'm gaming but if by some chance I lose sleep over it, I'll just open up the console and swap them myself.
EDIT: I reviewed the disassembled pics from the GCW site and I see the nub comes apart (to myself- well, DUH! How else did they get it in there?!).
Use this: http://www.amazon.com/Sony-PSP-Analog-Thumb-Stick-Replacement/dp/B0019LLGL6
Or this: http://www.traderscity.com/board/products-1/offers-to-sell-and-export-1/replacement-original-3d-analog-controller-joystick-with-cap-for-3ds-n3ds-308312/
As further evidence the Zero design is well thought out, it's worth mentioning the first reviewer on Amazon wrote that out of all of them, he enjoys the mushroom shaped one the most... Hey! Stop it! Get it out of your head!! STOP IT!