It's one of these bizarre things which says a lot about the British. Those who fly a lot, and who are familiar with various hubs, rate LHR highly. ( I remember T5 winning awards.) Those who rarely fly seem to only remember the bad publicity from several years ago and avoid it on the principle that things are always better elsewhere. Hence KLM have feeder flights from almost every UK airport, and Air Lingus from many.
It's not my most convenient airport to get to, but the only inherent thing I have against LHR is the delays. I don't enjoy sitting in the terminal watching the arriving flight do 20 circles over Middx on flightradar while wondering if I'm still going to make my connection at wherever. In particular, the terminal experience is much better than STN (my nearest airport).
So in short... the only thing I have against LHR is that it doesn't have a third runway...