I have travelled through several terminals 1/3/4 and find them all very different. Terminal 4 is out of the way south side of the runways but is modern and clean. Not a lot of restaurants inside a lot of shopping for that last minute duty free item however. Terminal 1 is old but once terminal 2 is redone it is expected to be replaced. Overall it is crowded but seems to be well taken care of. Terminal 3 is huge - if you have a flight from one of the ends of the long concourses take comfortable shoes! I think most North American and Asian carriers are based in terminal three so mornings are busy read maps to not get lost which is easy to do in this terminal. London Heathrow is a great airport a lot of things to buy or if you like plane watching all terminals offer a view of airlines from all around the World. It feels a lot closer to London than Gatwick but it can be a bit more expensive to get into Central London from Heathrow.