What about a product can irritate you?

CD wrapping. Its not enough to put the jewel case in some impermeable plastic wrapper, they have to put another piece of plastic across one edge that shreds when you try to pull it off.

hmmm? lets see...the dial until you win INPUT SELECTOR?! overshoots. undershoots. just doesnt stop! This is the sort of device that belongs on a safe and not a pre-amp. It may be odd, but in my ideal of an amp less is more. I would be happiest without a remote, indicator lights, and I prefer buttons to knobs where knobs are not necessary.

any unreliability.

When I had some Monster cables with their "turbine" connectors they loved to try to pull out the RCA's they were connected to. Gave those away to someone who I wasn't real enamored with - let them make his day!
Having black-on-black writing on the back of pre-pro's is always fun, as my close vision distance is getting more challenged by the day, and not having contrast on the input naming sorta completes the challenge. I got a couple of those miner style lamp headbands so I can make out the lettering.
Unreliable - Yamaha - my old DSP 3090 spent more time in the shop than my old Alfa Romeo, and Pioneer - I gave away my DV47 when it just decided to give up the ghost - too many times in the shop for that too. No more Yammie or Pioneer for me even for third or fourth systems.

My biggest are too-coarse or too-fine volume controls and tempermental remotes. The sticker on the CD edges also get to me so I echo Edesilva on that one.