I rarely if even use any London Airport (living in Manchester), but having transitted via T4 at LHR, the most stupid part of the design of the whole airport is the easiest thing to change - the security screening. Having arrived on a plane from TLV (supposedly the world's safest airport) why do I need to wait in line for an hour to change planes to fly to MAN?
I usually use FRA or ZRH, and their terminals are cleaner, easier to navigate and even with the split terminal design of ZRH, spending four hours there with young children was no trouble.