This is my biggest complaint with my local non-Super Center Walmart. Since it's so close to my house, I've been in there many times over the past year looking for a basic household item (e.g. hoseclamps, video game accessory, Christmas lights, oil filter, simple mounting hardware, etc.) and came back out with nothing. These are all things I swear I used to be able to find there, but no longer.
Seems like they never have what I'm looking for anymore. I attribute this to my local store's adoption of a greatly expanded food section, crowding out other merchandise areas of the store. I don't know who decides what they stock & don't, but they're a moron. Especially since I'd rather go to a full-selection grocery store for food, vs. Walmart, I don't see much reason to go to Walmart anymore unless it's for a REALLY common item I've seen there before, since they probably won't have it anyway.
That being said, my local store is very clean and organized and the employees are pleasant. I wouldn't notice reduced service, since I avoid "service" and sales people in stores like the plague.
Variation in Walmart "quality" is huge, since every Walmart is basically a reflection of the local community it's located in. In the larger metropolitan area I live in, there are nice ones & horrible, dirty ones. The latter are typically in older, more urban, or lower income areas. Since the vast majority of employees and management that work there are drawn from the local community, it makes sense that each Walmart will reflect the people who live around it. Centralized attempts at quality control would be pointless.