Guns Don't Kill People, People Kill People. But A Gun Helps
It is the recieved wisdom amongst many sectors of the roleplaying community that System is unimportant. It's about the group, they say, and the GM. A good GM can run under any system, a bad group can ruin any system. This is, as far as it goes, true. However this is no excuse for not creating a system as well, as tightly and as coherantly as you possibly can.
If people choose to play in bad faith, to grossly misinterpret the rules ("well it doesn't say you can't have a double handed sword in each hand") there isn't a lot that a system can do. But it can help, and that's what it's damned well there for.
You Can't Make A Good System Out of Patches
No system - not even a published system - is ever truly finished (look at D&D, Vampire and so on and so forth). However modifications piled on top of modifications just lead to things getting confusing and unweildy. Things wind up being The Way They Are because of rules that are no longer in the system, problems that no longer exist, or concepts that no longer matter. It is far better for everything to be dealt with at once.
A game has to work as a unit. The system - and indeed all of the individual aspects of the system - has to fit the setting and the way the game is intended to be run. Modifications to the system need to be carefully checked for potential knock-on effects. In particular, remember that LARP systems need to be remembered in the field, and it is far easier to remember one working rule, than one broken rule with six attendant patches.
Simplicity is a Virtue, But It is Secondary To Quality
A LARP system needs to be simple enough that everybody in a game can remember the important bits all the time. On the other hand it needs to be complicated enough to allow for proper variation in encounters and characters. Furthermore there are times when a little bit of complexity now saves a whole lot of complexity down the line. For example, the Through damage call could just be replaced by saying "none of the damage from sources X,Y and Z" is stopped by armour. Clearly, however, the benefits of not haivng to remember a huge list of "damage sources not stopped by armour" greatly outweigh the difficulty of remembering a single "Through" call.
The important thing to remember is that adding a layer of complexity to the system is only a good idea if it adds another layer of flexibility or verisimillitude to the game. The other thing to remember is that extra layers of flexibility and complexity should be, as far as possible, directed only at those directly using them. Additional rules for staff fighting and sneak attacks should be usable when pretty much nobody except the sneak attacking staff fighter is terribly familiar with them.
Here proper use of the pregame briefing is essential. You can include practically unlimited numbers of extra calls in a system, so long as you remind people what they are at the start of the adventure, and don't try to use all of them at once. Furthermore, if everything is kept simple enough to be expressed in a ten minute pregame brief, eveything should be simple enough for people to remember.
Options Are Good Provided They Are All Left Open
A well designed system should have multiple viable options for players and GMs, but there is a careful balancing act to be performed here. More precisely, there are two balancing acts to be performed here, the first being that too many options may overwhelm people, the second being that the more options you introduce, the more risk there is of one of them winding up being transcendantly more useful than any of the others.
It Does Exactly What It Says On The Tin
The rules should fit the flavour text. This is, I feel, the current problem with the Alchemy system. Although it is supposed to be about purity and transcendance and all that, it still ultimately boils down to healing potions. By similar token, the best way to create a character who is good in a fight should be to go with a straight warrior, the best way to create a character who knows about stuff should be to go with a straight scholar, and so on.