A few of my pet hates:
1. LFSs that label up common plecos with either just 'Pleco' or worse - nothing at all. Generally these are the same places that'll try and sell you it as something else with no mention of how large it'll grow.
2. Must agree with Darkstar over slate issues
I mean, why pay £3-£5 by the lb, when you can go to a local rockery place and get it for 99p for Great massive bits of rock and slate? You decide.
3. Customer-wise, ALL the stupid people who walk into a fish shop for the first time, decide they LOVE the marine aquariums and just HAVE to have a marine setup. On me and my girlfriend's searches we came across one woman who was basically saying to the LFS-man, "I don't care what you say, I'm sure they'll be fine in a normal tank, now I want to buy THESE ones" referring to the marine fish...I mean, how dumb are some people and how manby countless fish have died because of them? Or maybe these people only exist around where I live (?)
4. Shops where the fish are too small for sale... I only say this because one that we went to quite a bit has now started selling the tinyist angels (i mean, like a few millimeters) to the general public. I may be wrong, but surely that cant be right? Unless you really know what you're doing anyway.
5. People who answer your questions without even knowing the answers, like "Can I mix a Pirahna with angels and keyholes?" - "Of Course You Can, here's your receipt ,would you like a bag with that?".
6. Supermarkets (Even Pets-at-Home type places). I have seen fish in some of the most deplorable conditions. Certainly the ones I've been to anyway.
On a brighter note, I have to say, one of the BEST LFSs I've ever been to has to be 'World of Fishes' in East Grinstead, Sussex UK. From start to finish the best most knowledgable service I've ever had - especially for African Cichlids. THey will even turn people away and ask them to do more research before buying fish if they seem unready for the commitment. A tardis of supreme fishiness.
(unfortunately haven't been able to go for a long time as moved a while ago and too far away).