My biggest gripe at LHR is the difficulty in transferring terminals. Twice I've recently connected BA to BA to find it's T5 to T3. Both occasions the transfers took almost 2 hours gate to gate- which is pants. How can you build a terminal (T5) that isn't even big enough on day 1?
As has already been said, any slightest glitch and the regional connections get chopped.
Give me AMS any day.