As a rather new member to the cubing community, here is my list:
Cost of the V-Cube 5. I would like to own one but it is far to expensive!
Problems with the V-Cube 6.
Parity (I eliminate corner cube parity forever by orienteering the corner cubes before going to the top layer cross). Handy for even big cubes (4x4, 6x6, etc) and the 3x3 Void cube too.
Solving the last two centres on large cubes (6x6 and above).
Edge pairing (5x5 and above).
Not much else for me. I just solved my very first cube ever (a 3x3) just under 3 months ago and I'm very happy with my best times right now. And I have now fully upgraded to full 2-look OLL/PLL. I have already noticed the difference. I think I can get my 3x3 time down to 50 seconds pretty soon.