A quick discussion with Wolf-Snipe. A Xonotic player from Poland, aFPS veteran.
I started playing afps back in 3rd-4th grade in primary school with Quake3 (mainly playing with bots but also pretty ofter lan parties), but the main game that really start my afps history would be Quake Live back in 2008, when i really start to play against real ppl, i got ~1.5 year with CPMA starting in 2018, and Xonotic about 2 years now from 2017.
The most important reason was that a friend asked me if i would like to play it with him and i didn't really mind, i might be hooked by it at this point. Other reasons maybe are that from 2015 Quake Live begin to pretty rapidly becoming bad to put it lightly, starting off with sudden from the "browser-like" game, which was FREE-TO-PLAY (without any pay-2-win bs), with pretty good matchmaking/ranking system, good net-code, also adding a 2013 update which took it to steam which make it to have real "boom" of new players, change it into unnecessary Quake 3 clone to which you have to pay 10 euros iirc without those things that really made Quake Live good and different from Quake 3 at the same time. I wanted to add that i was generally bored with QL stiff gameplay and overusing hitscans.
This is hard to answer. For me it wasn't hard to get used to Xonotic from Quake Live, but if someone want to play it as their very 1st AFPS i will say it is harder than Quake Live. Because weapons firstly have 2 fire-modes, secondly no weapon functions are explained through single player campaign (I'm aware of Antibody's Beginner Guide where he explains most complex weapons and their functions) and i doubt without any "outside" help someone would be able to kinda learn literally all functionality of the "tools". Other thing that is hard to understand for beginners is combat, more precisely (expect ofc of affemation weapons) is movement and item picking up. By that i mean Xonotic is really, REALLY fast and most of "new players" can't handle the pace and even without speed they can't at 1st understand how they are supposed to move in combat (general dodging for example).
The slower pace of QL just makes it more accessible for newcomers.
Most importantly: spawnsystem, because it has some really bad tendencies to downright spawn you next to person who killed you or really predictable spawns on certain maps (aerowalk being the biggest contender in this category), visually i would like to see mappers start to use more real-time lights and shadows, because it just makes most maps look pretty "poor" in comparison to these which are using it (stormkeep, aggressorx for sample). Next thing is to put most options in UI and i mean even competitive ones. And as a moviemaker some sort of UI panel for demos (to simplify for example timescale[speed of demo]).