My pet peeve: Dive shops who do not list their pricing or other basic information on their website.
Price is not the only thing that matters, but it matters, and can vary widely (example: there was an $800 difference between the highest and lowest class prices quoted for my Solo class). Also, I should be able to easily tell (without having to start the online booking process) the price of boat dives and whether they include tanks & weights, extra costs for nitrox, etc. I should also be able to tell what time the boat leaves, where from, and approximately when it returns -- especially if I'm on vacation with non-divers, I need to know the schedule when selecting a dive op.
One reason this should all be on their website, and a related peeve, is that many dive ops are really bad about responding to emails/facebook messages. I've had messages that were never responded to, or responded to months later (after I went elsewhere). Also many dive ops that do reply seem to have Tarzan send their emails, as they are often very terse and usually incomplete -- like saying when the boat leaves but not the location, or vice-versa (I'm really only asking basic when-where-how-how much questions).
And I especially dislike asking basic questions by email and just getting an email back that says to call the shop. For various reasons, many people can't always call a shop during their open hours. If possible I usually avoid doing business with shops that make it too hard for me to get the basic info - it's really not hard to list the basics online. /rant