A couple of random thoughts in no particular order:
Unlike road bikes that have less maintenance, you must check some items on your mtb bike before every ride - Clean lubed chain (and gear shifting test), Shock pressure, Fork pressure, Tyre pressure and thorn etc presence, Dropper post pressure, Brake lever play and pad wear, and consider your tubeless sealant condition/level. Sounds like a lot to check - but it will become second nature.
Don't leave home without your Trail Permit Board !
Check the club website/facebook etc media on trail conditions. It very disappointing to get to a trail and find it is closed for maintenance etc
When riding alone - keep in mind that you need to ride more conservative and controlled. If you fall, it could be that bad one you don't expect - and it could be a long time before help or another rider comes past - furthermore, if you are off the trail you could be hard to spot.
Unlike road riding; you need to stand-up more to control your bike - example stand-up on tricky or rough technical trails or/and downhills
Practice using your front brake - it grips ! especially on a straight line
Don't ever drag your back wheel - it F@#$ up the trails
Brake before you get to a corner/turn - not in the turn - danger plus drag - don't F@#$% up the trails
Don't go riding single tracks if they are muddy/after heavy rain - itf @#$%^ up the trails
Take you own bomb, plugs, tools etc. - don't expect others to help you
Look at route maps to get an rough idea of where you are going to go - unlike road, most side roads will not lead to Rome - you will reach dead-ends - hike a bike obstacles - and angry farm owner and marshal abuse for leaving the trails. Plus, short cuts usually aren't
In spite of the above - sometimes you will get lost - often due to missing a trail sign, go back and check. If its due to confusing or missing sign boards - tell the next marshal you see how disappointed you are ! (but be lekker - don't get into a fight)
Look at maps for potential water restock points
Be lekker - even if its a paid event - you don't have the right to hold the trail line or hold up other riders
When you stop - get off the trail line to let others pass
When faster riders approach from behind, get off the trail line
When you legs give up on a climb, try not to get off - go to your easiest gear, lower your cadence and pace yourself - , it can be very difficult to remount/restart on steep hills - unlike roads, the surface will probably be loose
Relating to the above, opinions differ on who has 'right of way' - the rider going down a narrow trail or the rider coming up the trail - if its steep, let the rider going up keep going - but don't be a box, judge each encounter on its own - sometimes the rider going down has no safe place to stop due to speed, loose trail surface etc - and sometimes the climbing rider sees its easier if he/she stops. Say hello and thank-you.
Chances of falling are higher on mtb riding vs. road - Sooner or later you will fall - wear gloves
Sooner or later you will eat bugs and flies - don't cry - HTFU
Sooner or later you will get a Bee in your shirt or helmet - stop immediately or you will get stung (don't cry)
Momentum is your friend - unlike road riding, don't stop just because you went slightly off trail, just keep riding and re-join
Momentum ! - if you go too slow the rocks etc will try to stop your wheel
29er wheels are BIG, and with Momentum, your wheels will roll over most obstacles including rock gardens, river rocky beds, grass/weed lumps/tussocks.
Keep momentum for bridges over fences etc - (but not so much that you ramp/take-off ! ) And get in a gear that lets you add a little power and speed in case you go too slow at the top. The fence bridge is no different to riding over a dirt bump - with the exception that stopping on top / putting a foot down is likely to end in tears
There is nothing delicate in mtb riding - you must be confident and bully your bike over obstacles
There is no shame in taking the 'B' line ( the safer route at an obstacle, drop-off, jump etc)
Unlike road tyre pressure, hard tyres are not always better - depending on your route, decide on your tyre pressure before setting off on your ride
Always wear your ICE identification bracelet on a ride.
(don't ask how I learned some of these tips)