Why would people use a bus in downtown London when it is so congested? I can understand tourists who like the double deckers, but why would Londoners use bus when the Tube is (presumably) faster?
Mainly because it's slightly cheaper---London's bus fares, even after a 50% fare rise in recent years, are still exceptionally good value. You can go all the way from Ilford to central London on the 25 for £1.35, whereas the same journey would cost £3.70 off peak (and £4.90 during the peak) on the train/Tube.
A day's unlimited travel on all London's buses and trams is £4.20---the equivalent travelcards (tube, NR, DLR, LOROL) are substantially dearer. Children travel free on buses (so do pensioners and disabled people, but they also get free travel at most times on trains with their Freedom Pass.)
Don't forget, too, that it's a door-to-door service. To get to central London on the Tube I have to walk from my flat to the tube station (around 10 minutes, and crossing a busy road.) If I get the bus, it takes me five minutes to walk to the bus stop and hail a bus. It's slower (although not too slow, given bus lanes) but it's just as frequent as the Tube, I don't have to climb a long flight of stairs at either end, and I can alight two streets away from my destination rather than ten minutes' walk away. (On an open platform bus, it's even more convenient---if you're lucky, you can get off right in front of your destination if you're stopped in traffic.)
Having been on LT2 yesterday, there definitely seems to be something "off" with its climate control. It wasn't a particularly warm day, but upstairs, towards the front, I was roasting unnecessarily. I suppose we'll know when LT1 and LT3 arrive if this is a problem with this particular bus or a problem with the design of the air-chilling system.
Also, the conductors are unusually concerned about making you stand back from the platform edge while the bus is moving (which is a PITA when you're trying to peer out to see where your stop is!) Presumably this is out of a desire to avoid headlines such as
"BORIS'S BUS MURDERS FAMILY OF FOUR."