For me it's a combination of things:
- rarely having level boarding at stations
- badly designed toilets that always smell
The level boarding issue is at least being addressed with Greater Anglia's new Class 745 and 755 FLIRTs. Is level boarding found on trains in any other countries?
Smelly toilets also used to be common in some other European countries: the former East Germany for example.
Talking of toilets, in Britain older side corridor coaches usually had the toilet door at the end of the corridor (except Mark 1 Corridor Seconds which had two toilets opposite each other at one end of the coach just like Tourist Open Seconds). Early Mark 2 and Mark 2A Corridor and Brake Corridor Firsts also had the toilet door at the end of the corridor but Mark 2B, 2C and 2D FKs and BFKs had it in the vestibule (which AFAIK has always been the norm in mainland Europe). Not sure why it was decided to change to having it in the vestibule with the Mark 2B FK and BFK, or why they didn't do it like that on older coaches. I can't think of any particular advantage or disadvantage of either layout.
Mind you, on some of the German ICE sets that have compartments at the ends of the coaches the toilet door is in the corridor just like on Mark 1s and early Mark 2s. I also once travelled in an Austrian couchette car with a three-quarter width toilet compartment with the door in the corridor.
Some Continental couchette cars, and even some older ordinary seated side corridor coaches in mainland Europe, had a separate washroom with just a washbasin at the end of the corridor and a toilet off the vestibule. I expect that would have caught some British tourists out who were used to Mark 1s and who tried the separate washroom expecting to find a toilet!