Of course every LARP is different and it mostly depends from the people, then plot and some rules/system at the last. This are my experiences.
IF there are enough people who can play well talking characters, people in that field of interest will have fun. If there are people interested in battles, fighting and stuff, alike people will have fun too.
It is much more about the player skill (almost entirely), than the character. While in computer RPG you may know near zero about fighting, but your character be a soldier, in LARP it won't work at all. You need to have good condition if you are going to catch a thief or running away enemy. Exactly the same is about talking talking. If you aren't good actor, with i don't know... face expression, emotions, knowing the right words/almost language for the setting you play in. Your character will suck in it too.
The Plot may be really good, but it may happen there won't be enough organizators (like GMs visualising the world) for tens of people looking something to do. If course you may somehow gather players who are all capable of creating world, each will know some plot and the whole thing will be somehow going... But fact is most people aren't best in it and for enjoable gameplay organizators are needed.
Rules. There definetly won't be any dice rolling in the middle of a fight ;P Although characters may have some skills, which used at a certain moment may change result of this what has actually happened. Like you are normally wonded if you get hit, but if you hit your enemy right after he hit you, he is wounded instead. There is no probability for some action, there are only certain results depending on the player skill and some character skills.
Of course some people may decide they really need the dice rolling and there will be certain situations when they do so... but it generally doesn't work well.
The last thing is people respecting the rules. There are no GMs on every step and there is not much you can do if some guy just won't like the result and run away. It's all about people. It decide if the LARP will be fun or not.