I don't care how happy they are. I've been there, it can be pretty miserable work. What I hate is when they won't do their job properly. I don't expect smiles and simpering sweetness, but I expect comprehensive service from someone who is knowledgable about the store and the stock, and who will help me with any relevant problems or questions I may have. I don't want to be followed around, but a 'hi, can I help?' if I look lost/unsure is good service.
Strangely, I quite enjoy face-to-face customer service. I know not everyone does - hard work, low wages and all that - but I don't see why that is an excuse to slack and be rude. Politeness shouldn't be seen as a 'gift' that is given when in a good mood.
It goes both ways, though. I don't mind unhappy customers - everyone has bad days - but the snappy, rude, obstinate and difficult ones wind me up, even when I'm another customer. Sometimes it's deserved. I've met some really rude shop assistants who got both barrels and deservedly so. When it's just a default 'I'm the customer, I can do what I want and treat you like ****' attitude I find it incredibly rude and unfair. There is no call for that kind of snobbery.