Thought it was time to start such a topic. Here's my 2 cents:
These relate directly with the annoyances I get from the X4
- use black tape or black micro fiber cloth over profile light. 
the LED on receiver and the "home" button on wand are waaay too bright. When playing in the dark this is enormously distracting and even troublesome as it is almost the same as someone shining a flashlight directly in your eye. Make it less bright !
havent noticed - theG6 and G7 buttons are too close together and ALL buttons are too easy to depress. I can't count the times I accidentally depressed G5 and G7. Make the buttons a bit wider and firmer. I found the buttons on Aimon XB to be much better.
-G5 is too far in front. I have a short thumb and always have to re-position my hand to reach G5 button.And when I do, my fingers are too far on the left and right-click and I can't depress G7 anymore.
Controller layout is better
-for the love of god get the layout with the mouse (instead of a console controller) back
-and I can't stress this enough:
Wireless SUCKS!- errbody - MAKE THE MOUSE WIRELESS AGAIN !!!!!!!
I want this to aim and kill by itself, I don't want to have to aim at all. Make this change and I'll throw my money at a new version.