We'd get on at one station freshly showered and hydrated then get off at another drenched in sweat and ready to pass out. We used the tube extensively on our visit and it was by far the most uncomfortable (from an environmental perspective) public transport system we've used.
I'm not sure what makes the London underground so different from the systems in Glasgow (admittedly very small), Munich, Barcelona or LA. Perhaps it really is so much deeper under ground.
That said, the contactless payment system was fantastic. Being able to use my credit card to move around the city without buying a separate ticket was very convenient.