This is potentially such a large topic, as bars, pubs and clubs are obvious flashpoints of cretinous behaviour, but I'm going to focus on the 3 main ones that annoy me when attempting to buy a drink from a busy bar.
Ordering one drink at time. This one has got to wind up both staff and customers alike. Somebody ordering one item at a time, making the staff member walk back and forth behind the bar delaying the whole process. Similarly, people who can't remember the whole round and have to shout back to their mates to ask what they wanted, while the staff member has to stand there like a lemon. When you order a round, you should get one attempt, tell the staff member everything in the round, if it's too much for them to remember they will say that, or come back half way through and ask for clarification. If you forget something once the order has been placed and all the drinks are infront of you, its too late, queue up again.
Not having money ready. Kind of similar to a point in my post about supermarket checkout etiquette. You know roughly how much a round is going to cost, so have a note out ready to pay for it. Anyone who is "just getting rid of their shrapnel" should have the coins ready to hand over. Not make everybody wait while they drunkenly try and count coins out onto the bar.
And finally, people that stand drinking at a packed bar drive me mental. While everybody else is clamoring to try and find a space, these people are blocking up the place, either because I suspect, they can't be bothered to find a table elsewhere, have no mates, are trying to chat up the barmaid/man and/or any female/male that stands next to them, or are so alcoholic they are reserving their space at the bar because they can't bear to spend time queuing up again. If you ask them politely to move so that you/others can get to the bar, it is not often met with approval. Arrogance defined. "This is my area of the bar, everyone else must move around me." Whatever the reason, it's just common decency to get your drink, and move away from the bar area as quickly as possible, allowing others the space to get served
