While I don't personally hate LHR with a passion, I find the fact that you have to go through a (badly run and always understaffed) "security" screening on connecting flights a complete hassle and one where your oneworld status is not recognized, and the additional "conformance" check and associated lines an absolute head scratcher customer dissatisfying bureaucratic hurdle. Don't get me started if you're arriving and departing from T5B: they train you to T5A so you can line up twice for "conformance" and "security" and then train you back to T5B. And if it snows 5cm, then all hell breaks loose.

Personally, I think LHR as a shopping mall rather than a connecting hub. As a mall, it's very pleasant indeed, but as a tool for facilitating travel, not very. Would rather connect through AMS or ZRH every day, or any of the US hubs.