While the situations that the staff landed themselves in each week were always amusing, what made them so funny was that in every case each character remained true to themselves, and always reacted by doing something that was fitting to their character as opposed to what would just be a funny thing to do. In turn this made it even funnier, and eventually some catch phrases and long running gags were established such as:
Karen never getting anyone's name right. The more angry Karen got, the more TJ wound her up with unfunny jokes. Debbie's engagement to a guy called Derek which she was constantly calling off. Freddy having a superstitious Scottish legend to tell for every occasion (Such as the Legend of the Great Hairy Bogey.) Dexter continuously sucking up to Karen with no success. Customers constantly being ignored or mistreated because they were getting in the way of good story lines!
This show was very well written, and while it was ultimately a children's show, some of the humour, especially between TJ and Karen could be quite adult at times. And as someone already mentioned, you could cut the sexual tension with a knife.