That means that the use of the system on lesswrong is based on the assumption that it will only be used by people who are much like you and will therefore work well for you. But given that lesswrong is an open system, will it always stay that way?

Well, nothing lasts forever, supposedly. If in future Less Wrong's quality gets diluted away, it won't matter to me if it keeps using the vote system or something else because I won't care to be on Less Wrong any more.

However, part of the function of the vote system is selection. To put it brutally, it drives away incompatible people (and signals positively to compatible ones). So I think LW will stay worthwhile for quite a while.

And yes, in a way this is one of your negatives from your other post which I actually think is a positive. If someone gets consistently downvoted, doesn't get why, AND can't ask and find out and update on that, then with some probability we can say we don't want them here. I'm sure we lose some good people this way too, but the system's better than nothing; at least what gets through the filter is much better than things would be without it.