• QSS has a revised network protocol, and will use it by default.
If you wish to record a demo to play in regular quakespasm/etc, then please set cl_nopext 1 before connecting/starting a game.
All QSS demos should also be playable in FTE, the reverse is not true.
• QSS supports master servers.
If you wish for remote players to be ale to connect to your server then set sv_public 1. Otherwise set it to 0 BEFORE you start a map if you want to prevent unknown players from finding your server. You may also need to configure your router to forward inbound packets to port 26000 on your server.
Use sv_protocol 15, 666, or 999 to select the base protocol (15 allows all vanilla nq engines to connect, along with its limitations.
• QC Extensions:
If a mod malfunctions in QSS but not QS, then try using pr_checkextensions 0. This will prevent the mod from being able to autodetect any qc extensions, and thus result in the same behaviour. If this does not work then please report this as a QSS bug.
Note that this may also break any mod that depends upon those extensions.
Check qssextensions.qc for descriptions and a list of what's supported.
• Particles
If you hate any map's use of rain (or other surface-emitted particles) then set r_part_rain 0. This will not affect weapon impacts etc. For that see the previous thing to note.

• Framerates above 72
Quake's physics are known to be glitchy above 72 fps, and stuttery when running at 72 fps on a 60hz screen (use vsync!).
If your monitor runs at a rate higher than 72 then you will likely want to boost your host_maxfps value, or just set it to 0 to uncap framerates. Networking and physics will still run at 72 fps.
• Arcane Dimensions
AD spams stuffcmds before clients are connected, and can overflow buffers before the client is even meant to be on the server.
Set sv_gameplayfix_spawnbeforethinks 1 (in ANY engine that supports it) if you want to host a multiplayer game of Arcane Dimensions. This is still required for ad 1.7.1

• Non-vanilla Behaviour
QSS is based upon Quakespasm and thus inherits all included non-vanilla, so:
• View Angles
Quakespasm has an increased view range, which may be considered a cheat, and certain servers WILL clamp your view, resulting in jerky or quirky views.
To correctly match vanilla, you must set: cl_maxpitch 70;cl_minpitch -90
This is recommended for anyone making maps that are not meant to be exclusive to Quakespasm, or anyone who wants to avoid claims of cheating.

• Model BBoxes
The size set by setmodel for non-brush models is well-defined in vanilla, in that it ALWAYS sets the size to +/- 16. However, in Quakespasm the size depends on the mdl itself, and can change with replacement models etc. This can result in entities being off-ground or falling through the floor.
Set sv_gameplayfix_setmodelrealbox to 0 for the vanilla behaviour. If this breaks your mod then you'll need to fix it with extra calls to setsize. If this breaks your map, then remember that that's what other-engine users will see.

QSS is not intended to change the aesthetics from that of QS (beyond more file formats supported). People who want more graphical effects are probably already using an engine that supports them, and implementing them in QSS is likely to drive users away instead.
QSS does change the networking (I consider this a bugfix due to how dire parts of vanilla quake were), but not in any way that should change the normal behaviour of the game's physics nor be visible to existing mods.