Too much soap - when peeps stay here if they use a top loader - I tell them not to bother with soap on the first load as they dont realize how much soap their machines leave in their clothes. In almost every case there is so much I'll come close to suds lock.
Improper or no sorting - classic college load, or people who constantly look like their stuff is all grey.
Everything mixed then washed cold - mixing rancid underwear, socks, animal cleanups, with kitchen rags, napkins, face cloths. It's ok to mix these things if you can sustain high temps, or use bleach if you cant but otherwise thats just gross.
Too much softener - people use way to much and it can start to gum up machines.
Scent beads - wife loves this crap and her test of cleanliness is one metric - smell. Could be completely dirty but to her if it smells good its clean.
I hate dryers that don't reverse - king duvet balls up frequently.
Washers that use too much water.
For every drop I use I've got to pump it, filter it, soften it, filter it again, then pump it again using a transfer pump to the sewer system. I have excellent water, but need to maintain this system, and waste is expensive.
On a front loader if I can see water pool during the wash it's too much.
Rinsing - sure go to town with the water.
Cheap Bad Soap - I have dry skin as it is, crappy ineffective soap really lets clothes build up grime.
I could go on and on, but Im a total laundry snob.