Attitude, helpfulness, thinking on their feet etc - different supermarkets want their staff to act in different ways to how other supermarkets do.
In some supermarkets, staff are encouraged to walk their customers to the particular shelf if they ask where something is, at other supermarkets, the staff are encouraged not to do that, and just give directions instead. That's one example. Others are such as what they want staff to do in other scenarios, such as a spillage, breakage, customer accident, etc. Or what they expect their shelf stacking staff to do in other more relevant scenarios, such as if there's no space on the shelf, what they'd do with the stock they couldn't put on the designated shelf space of if they see a shoplifter. Or importance of checking best before dates and rotating stock, putting items "face front" on the shelves, etc - again, different supermarkets have different ways of operation. Checking "shelf edge" prices and weight against the stock they're putting out, etc.
Also with the increase in use of computers, hand held tablets, scanners, etc., different supermarkets may have different systems in use and want shelf stackers who are more computer literate to work the devices quickly and accurately.
Lots of variables really between different employers. And there's a lot more to "shelf stacking" than simply putting stuff on shelves!