I've only been to Paris a handful of times and while the system is efficient, a lot of the stations and trains on the metro did have a big problem with human waste. If you see a nearly empty carriage on an otherwise busy train, someone might have done a shit on the floor - learn from my mistake and board another carriage.
London's not without its problems - chiefly for me the ear damaging noise and heat on the Central and Vicky lines - and the awful state of some of the rolling stock and track due to funding decisions made by the central government leading to delays and cancellations. This is extremely bad on the Central line right now, which hopefully changes following the recent election. The liz line, overground & DLR however are pretty great, and other services like Thameslink are pretty good with the exception of poor timetabling on some branches.
