What was the design choice behind going with abbreviated URIs for topics (/t), and categories (/c), but using the full form for accessing user pages (/users)?
Because /users is a page on its own whereas /t and /c aren’t 
5 « J'aime »
The average end user isn’t aware of that though, just seems inconsistent.
The average end user isn’t looking at the location bar.
12 « J'aime »
Shorter URLs, less desire to “localize” URLs since they are not full words.
3 « J'aime »
Probably also because users don’t create super long URLs while topics do. I’m glad to have a convention that keeps these a bit shorter.
2 « J'aime »
We need /u/ for /users/ 
6 « J'aime »
Agree, but we should open a specific topic to track this
4 « J'aime »
C’est désormais le cas et implémenté sous la forme /u/
7 « J'aime »
Je peux affirmer avec certitude qu’aucun de mes utilisateurs n’y prête attention, lol.
4 « J'aime »