There are only two issues which bother me.  Lighting and Memory management.

It would be great if the lighting intensity in the preview bore some slight relation to the final render, especially with IBL.  I know, most of us know to "turn down the lights", but it's frustrating to have to render and render with minor adjustments to get a realistic result.

Memory management - well I've learned to wait until everything loads before daring to move my mouse, otherwise I snap back to the desktop.  A reboot frees up the memory, but it's hard to remember to reboot when deep in a project.

That's it.  Everything else is fine with me - I don't use the hair room, and am constantly learning in the material room, but that's half the fun.

Lotus 123 ~ S-Render ~ OS/2 WARP ~ IBM 8088 / 4.77 Mhz ~ Hercules Ultima graphics, Hitachi 10 MB HDD, 64K RAM, 12 in diagonal CRT Monitor (16 colors / 60 Hz refresh rate), 240 Watt PS, Dual 1.44 MB Floppies, 2 button mouse input device.  Beige horizontal case.  I don't display my unit.