Hm, I should make a list of these things. Reaver, care for a joint effort to help put these things together for fledgling GMs? You got way more experience in doing things right, I got mostly experience in doing things wrong and trying to fix them after. =P
LOL, the only experience I have is experience at what NOT to do...

But I have learned those lessons well.
It was a long and painful road of almost 38 years of tabletops and GMing/playing. And I have/had the advantage of having a great GM for most of my time in Shadowrun that
I could learn from for my own games.
But yes, I could help write out a small list of things to keep in mind for Shadowrun.
If fact, here is the first, "On" and "Off" tip for new GMs and Shadowrun.
The world is a realistic one Yes, its a game, and yes it is a fantasy setting. Yes there is magic and super tech all around. This is not what I mean. Many players and GMs that come from other systems have a hard time grasping the differences that the rules of Shadowrun, and just how much "power" characters can and can not have.
Many other tabletop systems have a "leveling" system of some sort. After you gain a certain amount of points your character goes "ding!" and your character gains a bunch of nifty new bonuses. And it also gives you an at a glance view of how powerful a character is. After all, a LvL 5 character in going to be "less" then a character at lvl 15 in a given system.
This is not what Shadowrun is, or is about. Unlike in other game systems, In Shadowrun it is entirely possible to have a freshly minted character to run with a bunch of old pro karma whores and still have fun!* It is this very lack of levels that makes Shadowrun realistic.
In other games. The group goes running off to a dungeon, kills shit around their level. Go "Ding!" and leave. They go to the very next dungeon, and magically find shit to kill that is around their level, go "ding" and leave. Rinse and repeat. (yes, yes, "very simplified!!" you scream, but the point stands). But in Shadowrun, the group goes down the barren kills a bunch of gangers. and... gain nothing, except what they salvage as the very act of "killing" in SR doesn't give you anything to your advancement - completing team objectives does! And if that team went off and completed a bunch of team objectives gained karma and then came back to that street corner?? They would find the gangers... and they would be even easier to kill!
Unlike many other settings, in shadowrun, what you see is often what you get when it comes to enemies. A Ganger, is a ganger, is a ganger. And she will always be a ganger. Why? Because she's a Ganger. Now what do i mean by that? lets look at the CRB to highlight this point:
CRB 382 lists the base stats for Gangers. And what you see should be what you get when you encounter them in the world (Attributes adjusted for metatype, of course)
The base skills of:
Blades 4, Clubs 3, Etiquette (Street) 3 (+2),
Intimidation 4, Pistols 4, Unarmed Combat 3
Should never change. Why? Because they are a
Ganger!
To highlight this lets look at look at an elite special forces of page 384
Athletics skill group 7 (10), Stealth skill group
6, Close Combat skill group 8, Demolitions 7,
Firearms skill group 9, Perception 7
Seeing the difference?
One is ganger, with ZERO professional training, ZERO resources to spend on equipment and education. the other has had the very best that their agency could afford.
Now why do I bring this up? Because the very first concepts that many new GMs struggle with is the concept of "challenge" in Shadowrun, and figure that they have to play with the numbers to keep things
interesting for their players; often because they are looking for that magical Level number that tells them how powerful a group is.
So they think they have to start adjusting skills to compensate... And soon you have gangers that are starving in the street, that any military force in the world would pay a 5 digit salary to have in their ranks as gangers start sporting combat skills in the low teens!
No. As has happened every single time that police force/military pushes back against gangers, you end up with a lot of dead gangers! As is the same in Shadowrun... you throw gangers up against a combat party of runners... expect a high body count!
On the reverse side. What the in the name of FUCKING FUCKITTY HELL are those gangers doing picking a fight with a Runner team?!?! Gangers may be the pond scum of the streets, but they are not usually suicidal! (Ok.. Yes the halloweeners come very close.... but still!) They should beating feet about .25 seconds after the troll Runner turns the first ganger into paste with a smile!
People in SR do things for the very reason that they do things in our world, because they want to live, be comfortable, and happy. And just like our world, not everything is going to stop you from achieving those desires. But, unlike our world, there are not as much helping to achieve those desires. There is no social safety net to catch those that are unemployable. There is no real government programs to help with the many things you (the reader) have come to expect. No welfare, no healthcare, no social pensions, underfunded public education, spotty utilities, and more taxes. - But this is all covered in the history of the game... which should be required reading!
*I say this because I have seen this happen several times. I play in a living campaign with 5 steady players and the same GM for the last 25+ years... Occasionally over the years people have joined and dropped out as time goes on. Those who join always start with "fresh characters" or their old character where they left off. Which has lead to games where there are characters with 12,000 karma teaming up with people with 0 karma! And at the end of the day everyone who played enjoyed themselves!
I admit this is something that takes a huge amount of expertise and skill on the behalf of the GM, and is not something that I have been able to replicate with the same proficiency.... I just don't have my GM's skills weaving that complex of a narrative together and make it work as well as he does.