Well, large bare spots on GS/coco fiber backgrounds suck, but sometimes you have to leave it blank because it'll look like there's too much going on, or a plant will invade the area and cover it anyway, rendering your work useless.
I dislike people using home depot/lowes plants
I dislike when people do not take the time to put black silicone on the glass BEFORE they apply the background
I dislike when people do not hide their false bottoms by making a box out of the false bottom and filling in the gap with stones
I dislike pothos
I dislike tanks filled with grapewood
I dislike tanks with chunks of pillow moss everywhere rather than leaf litter
I dislike tanks where people attach broms to the glass with suction cups
I dislike tanks where there are WAY too many plants. I mean, I get that we're supposed to be "recreating the jungle," but at a certain point, looking at that jumbled mess becomes an eye sore. There's a fine line between good planting and too much planting, and I see it overdone time and time again. Raf's build is a perfect example of good planting because you still get to appreciate the background.
I dislike tanks that are set up for the wrong species. I mean I'm no expert by any stretch, but I know a few things about setups as far as what is beneficial versus useless for a particular species
I dislike tanks that have nothing going on at the bottom of the tank - it's just perfectly flat, no logs falling over or wood sticking out or dynamic shaping of the soil into hills, crests and troughs
I dislike people placing tillandsia in places that give you the impression they'll survive there, when in reality, you know they'll rot in a week
I dislike people covering the back and both sides of the tank COMPLETELY. I get the whole "security" feeling for the frogs, but it makes observation seem...limited.