speaking as a general arduino hobbyist.. that you are targeting..
Most of the NEWER people arent going to care (or understand the hardware behind.. what it allows or restricts because of it)..
Hardware/usability wil be #1 in my opinion..
so if its a kit.. needs to be nice and easy (like the Adafruit kit is)
I would say the software/library would need some REAL help...
I know there are samples for the WaveShield.. and they arent THAT hard to understand/follow.. but they could be NOOB freindly.. easy to use and understand class functions/methods..
etc..etc..
SD or microSD?
What type of DAC? I think the Waveshield libs only work with that specific DAC... so that would need to be 'revised'??
Have you given any thought of its useage? might help form some of the functions on your end?
Seamless looping?
What about layering or mixing audio samples? (loop in background SEAMLESSLY playing.. with layer sounds over it?)
Im assuming this will take all burden/processing power off the Arduino then? I believe the sampling from SD card takes ALOT... and has the SPI bus pretty busy as well? (with Arduino/Waveshield)