I've reported many things over the years, particularly to TfL - graffiti, wheel flats, possible track fault (at Leystonstone, it was dipping by a few inches as train wheels passed over it), out of date posters, illegible bridge identification notices, audio announcements going backwards (a 350 from Birmingham to Liverpool thought it was travelling from Liverpool to Birmingham), door buttons not working (at least three times on the DLR), very dirty trains (Piccadilly line more than once), missing/broken station signs, a whistling noise on a Chiltern 165, etc. Most recently a very broken PIS at Llanfairpwll (it would flash on for about ¼ second every 2 seconds but was otherwise blank).
Mostly no response but I have had a few, occasionally positive.