BAA spends £1 million a DAY! on new construction at Heathrow.
T4 to me has always seemed the best of the LHR terminals - it's easy to get to, easy for check in (that huge hall has loads of space) and then when you get through the passport control the shopping, gates are all well arranged. The only prob is the mammoth walk you have to get to those 'cul-de-sac' gates.
T1 is much better now that it was redesigned airside, but landside the checkin areas are still a mess. Domestic lounge is much better now it has been tidied up.
T2 - haven't been there for a while but apparently it's very nice since the 1996 refurb.
T3 - a bit of a mess really. I think this has to be the worst LHR terminal. It's crowded, the check in area is crap and the duty free/lounges are all a mess.
Terminal 5, when it gets built, will be incredible. There is also a plan to build another Terminal (6?) in the opposite side to T5 (i.e. the maint base) and then extend the satellites all over the central area (which would involve demolishing T1, T2 + T3.)
As soon as T5 is complete BA will transfer all their ops there (Int, Euro and Domestic) which will make transferring at LHR as seamless as at AMS or CDG.