Hi, all, first post, been lurking a while.
Actually been playing with this concept for a bit, and I have one other thing to add to the discussion:
http://en.wikipedia.org/wiki/MezuzahMy partner-in-crime/sounding board and I have been discussing this, and agree that it the mezuzah was put up by a devout member of the faith, it would literally function as "Bless This House" and boost the threshold level by +1 or +2. (this only works if the parchment is regularly checked and then reattached with the same level of devoutness; if the parchment is damaged or if the sullen teenager does it just to get it out of the way, no bonus).
But as for the main eruv discussion, having grown up in an area where the eruv has real meaning and significance (I've seen 60-year-old men put on their heavy woolen vestments at home and
walk the distance to synagogue, wearing them on top of the stereotypical black suit... in the
summer, rather than break the prohibition of carrying on the Sabbath), I'd agree, yeah, most Eruvs are not going to be more than +0 thresholds, because the community its servicing is too new, too divided, or the eruv is too frequently down due to inclement weather breaking the boundary string. But there will be a few that are more potent. And some that will be
very potent.
A suggestion for a rule of thumb? +1 threshold strength for every decade after the first that the community has been there with at least ten families (a
minyan), as a base strength to start fiddling with. Other factors effects--population turnover, number of synagogues, etc--can't be rule of thumb-d that easily.