I haven't encountered any issues as yet with most of the stores that I shop with (returns/complaints, etc), nor have I had to contact any online-only stores, so not much insight to offer there, but:
The staff at Next make me furious, each time I visit the store they hover over me as if I might be a shoplifter. You feel as if eyes are upon you constantly, and sometimes it has made me simply walk out of the store. They approach customers far too often in store, touting the catalogue and asking for personal details, etc. Just so annoying, like it gets in the way of your personal space, if you know what I mean.
And their online catalogue is a serious joke: at least 5 times I have never received my goods, only to be told much later that they hadn't been in stock when I ordered them. The checkout, of course, told another story.
The absolute WORST offenders for customer service, in my experience, have been electrical stores, like Curry's, Comet, and co. Not fitting for this thread, but I am permanently baffled by the inexperience of all their staff, who know next to nothing about the products, and occasionally look at you as if you are insane for expecting them to. Oh, and in such places, they still think it is a doddle to fool one of the fairer sex with smooth talking or merely treating you like a silly little girl