Sales tax is one thing. Not getting price raped when I want to buy a 6 ft Ethernet cord or HDMI cable is another. And sadly, they rarely have quality merchandise in areas where I'd rather buy from a B&M store, like monitors. To top it off their customer service is pretty lacking. In fact the last time I went to a Best Buy, I wanted to try out a Blackberry tablet that was on sale. They had one hooked up to the display kiosk. But it wasn't working. And both of the Best Buy people I talked to shrugged their shoulders and said, yeah it doesn't work.
Also, I used to talk to a guy that used worked at Best Buy and those guys didn't give a crap about the things they unloaded from the trucks. That's not to say that online sellers always package things well. Newegg has let me down with their packaging of PSU's.