As some of you know, the 5th ed. is very similar to the Magic World game Chaosium is selling now, since they no longer have the Eternal Champion licence. Most of what I write here is probably applicable to Magic World as well. Stormbringer often struck me as a great base for a generic fantasy game, and Magic World looks to be just that great game. One day I'll have to get hold of a copy.
SystemThe first thing about running this game is how to approach any BRP game. Everything is a percentile roll, either against a skill or against a multiplied stat. Anyone can do whatever they like, and as a GM you either make up a percentile chance of success or picks a skill/stat. Anyone understands "You have 65% to success, roll the dice". It's newbie friendly.
The second thing I noticed was how many fiddly bits there are when you look beyond that basic concepts! Some of them have changed in different editions, and I'm not too keen an all of them.
StatsI understand why you might roll 2d6+6 for stats, since it makes the characters more heroic. But, I think those really oddball stats that can happen in a straight 3d6 bell curve are usually my main hook for roleplaying, so I'd keep the 3d6 method. If you like your game more heroic, what you want to keep an eye on are probably hit points. This game system is really deadly! I suggest calculating HP as CON+SIZ if you want your game more heroic, instead of the 2d6+6 stats.
Combat rulesFirst off. This is a deadly game! With the major wound rules, you can't just add up hits and keep pushing on. Even smaller wounds will hurt as they pile up enough. I liked the idea of your character falling over after a certain amount of rounds after taking a major wound. I am less certain about the adding up of lesser wounds. Why do you have to make a POW x 4 roll to stay conscious when they add up? I prefer the Call of Cthulhu way of rolling CON x 5 not to keel over. I'll probably do that running the game again.
In 4th ed. Stormbringer you had separate ratings in attack skill and parry skill with a weapon. I kind of liked that, and the idea of a "finesse" fighter focusing a parrying and feinting before lunging for attack. They kind of open the option for you in the book to add your experience either in attack or parry. Another thing I like about the Parry/Dodge rule is that they are actions you can do over and over again. It makes for a more fluid combat and being able to dodge all attacks (if you have a really massive Dodge skill!) is probably good considering how easy it is to be eliminated.
MagicIn the game I ran we didn't have any summonings. Earlier editions of the game only had magic based on demons and elementals. Editions after the 4th added some other variants, and those round out the system to cover more kinds of magic. Apparently it's supposed to better model the Elric stories as well. Frankly, I only remember the summonings, but it makes for a better game engine to include more options. Friends of D&D will even feel at home with the basic spell system, since as long as you pay you magic points the spell will go off and there's no roll involved.
I kind of like the idea of introducing some randomness in spellcasting, but running Stormbringer I'll do it by the book. If nothing else, call it a concession to the potential players coming from D&D. If you'd like more randomness and making spells less common, make each spell a skill. That way you'll get some drain of build points, and randomness.
AllegienceThis something that was added to the rules after 4th ed. I was never really happy with the former "Elan" system, but was unsure of tracking points for Chaos/Law/Balance as well. Now in the 5th ed. they start to mean something, as you can "cash in" those points for extra skill points, hit points or magic points. It can be used to give some flavour to the game, involving the players a bit in the cosmic battles.
I usually say that alignment causes brain damage, as I've seen smart and intelligent people reduced to 12 year olds by it. Everyone remember how you ran your first game, misunderstanding most of it and clinging on for dear life to those rules that give some kind of focus and you think you can use to beat the game into shape with. It's quite natural, but then you grow older and relax. Sadly alignment brings that back out and people who are usually sure of them self and have both wife and a job are reduced to whimpering 12 year old kids who can't make a moral choice of their own. Luckily, allegience is not prescriptive.