My pet peeves are things that should be fixed in ANY render and that is pokethrough of hair into the mesh of the clothing, figure or props.  Drives me batty almost consistently that one does.

Never ending scrolling promo pages, if you need more than the 4 slots allocated then please hyperlink thumbnails and give us a break from interminable scrolling.

Postworking on promotional store images, compositing yes, you HAVE to but postwork should be a complete no-no, excepting painted hair on a character or clothing promo but no to filter effects and hue/brightness adjustments..

Lack of shadows in any promotional image puts me off, spend the extra time to render something that looks like it has a little depth and like you actually care about it.  If you care I might just.

Little to no promo text drives me mad as well.  Do i need another product to use your item?  What does the item do?  What exactly is included?  Tell me up front don't make me guess from the promo images or pull up the read me in hope that the file list will enlighten me - you'd be amazed how many times I have passed by a product that looked moderately interesting in the promos because there was little to no information entered in the promo text slot.