So small
The train sizes are pathetically small. This disastrously includes new lines such as the Jubilee line. Compared to a city like Tokyo where the train sizes seem 3 times larger (though for a city with twice the population), it just doesn't seem right and I imagine this will be repeated. The flip side is that it gives a frequent service but this means more drivers who aren't really needed. The height of older trains, such as the central line (especially around the doors) is also lacking, but believe it's better in the newer trains.
So noisy (and hot)
It's crazy how noisy parts of the network are. It wouldn't surprise me if a lawsuit appeared from someone with damaged hearing. But even from a customer's perspective the Central Line is already very hot (in summer), often very crowded and in some sections very loud, it's in no way enjoyable. I can't help but feel that there must be solutions that aren't extortionately expensive, even if it's only a mild mitigation. Related: https://www.bbc.co.uk/news/uk-england-london-42791299