There are a couple of problems with the request that the description be more concise, I think.
First, the game itself is so open-ended (Not a bad thing!) that you NEED to establish ground rules before every game you play anyway. Writing them all up-front is actually a good thing, since it saves you from having to come to independent agreements at the beginning of every round. It's an effort to save time, not an arbitrary set of restrictions.
I'm not necessarily saying they should not have these things, just that it makes much more sense to codify them and link to them. It will make an announcement seem much less likely to be perceived poorly.
There are also ways to make restrictions more palatable for the uninitiated (as a modern 40k player is going to have NO IDEA why he can't use an unbound army or a set of Imperial Knights for his force, and the rules telling them they can't just make them assume they don't belong anyway). If it were me I would combine all those restrictions into some kind of special Battleground army that gives the options as a Force Org of some sort. It says all the same things in one graphic that half the tournament announcement says, and makes what is currently a stern restriction instead seem like an intentionally thematic element.
Look at your own description of events: "a few rounds of 40k with scores for winning, playing nice, and looking nice."
The goal of an event should be to attain this effect with as few changes as possible to ensure a level playing field.
Scores. How will they be scored? It can't be simply subjective, since that's not much fun and not very fair (think of Gymnastics or Dancing or Figure Skating--there's always a huge argument about the quality of the judging). For objective scoring, you need some kind of standardization.
We have one. Victory points!
I've never quite understood what exactly is wrong with "whoever has scored the most victory points at the end of all three of their games is the winner"...Tournaments have this burning need to insert their own third-party points systems to tinker with this, or to ensure that victories are not equal or that near victories are as worthless as crushing defeats (the much-vaunted Battle Points).
I attribute this 100% to a simple lack of changing with the times. Tournaments had their points for win/loss/draw structure because originally there WAS no universal point in the game. Now we do, and people simply do not want to adapt because there is a social stigma. There is really no need to standardize anything beyond making any other things you want to factor for a victor (like painting or sportsmanship be converted to more Victory Points).
I understand why it isn't used 1:1 of course (this introduces the other issue, where you have players who "win" all three of their games while someone who lost one or two but got a big crop of points comes out ahead...but you can still do the win/loss/draw format without an issue while still using Victory Points to differentiate somehow).
Trying to turn this format into something more modern is something of a morbid hobby of mine, but I've quickly learned is all that seems to do is make people go "Dude you tournament wrong" (which is the reason I have opted to never, ever, EVER run another such event at the Bunker).
Scoring, and transparency about it. I have mixed feelings about it. I'd rather the scoring be presented to people who sign up, instead of being presented beforehand -- people should want to play first (I know this isn't feasible, of course, but a guy can dream). People show up to WIN when you hand them the rubric and tell them how to win. Without this, people can only show up prepared to play a few games.
People min/max, not just the lists they use but the event itself -- take the recent 750 tourney announcement, or any BG event announcement. It is almost universally followed by seasoned pros finding one or two clever gross builds that supercede the announced restrictions or take advantage of what few things are allowed. Adding more restrictions just adds more of those wrinkles to play with.
Then again, I'd like to see some kind of event that encourages people to actually have fun, and that kind of thing is too subjective to codify (though I'll continue my experiments at Evil Dice but I realize they are just that -- my own morbid tinkering that will never be supported anywhere outside my own four walls. Fun is absolutely 100% the last thing on any serious tournament organizer's mind. All they are generally doing is trying to avoid getting bitched at -- which is futile, so I don't know why they bother).
I want to stress I'm not saying that all this actually change. The Battle Points, the up-front restrictions, etc... that stuff obviously isn't going to change any time soon. Just that it could change, in ways more universally in tune with the current generation of new players/new gameplay.
Second, if you don't post that stuff up-front, people WILL miss the info and show up with something unsuitable. Heck, I'm sure a number of folks already don't read it carefully! My day job is as a technical writer and trainer, so trust me, you can never reach everyone even with the simplest words and obvious presentation.
This is true, of course. But the goal isn't to reach more people, as much as not to drive people who already see it away. Plenty of sets of eyes see the announcement. It should look more like an event announcement, and not a proclamation of how to play. And of course, playing devil's advocate, there is always the fact that if you don't restrict anything, no one can accidentally violate the restriction.
There is a lot to be said for presentation. An announcement for a 40k event that has maybe 20% going "Hey we're having an event here's how to sign up and what you need" and 80% a wall of text telling everyone a number of things that are changed sends and reinforces a message -- "You must play like this or you should just not come."
BG is in q unique position to craft the format of tomorrow instead of just emulate the formats of yesterday, and I guess that kind of skews my thought process quite a bit. I don't know, man. Just spitballing here! It is good to see some discussion that is not disruptive. Keep up the good work, all.
TL;dr:
1. Too much emphasis on the changes seems like it isn't going to attract many new players;
2. Maybe there are ways to make the events seem more inviting without changing the core idea
3. This post is roughly comparable to the tournament announcements...so if this post is too big for you to read... 
