I also like that the folksonomic approach (as in, there are no “pre-established groups”) allows for a great deal of expression, of negotiation (I imagine that #barcamp will be a common tag between events, but that’s fine, since if there is a collision, say between two separate BarCamps on the same day, they’ll just have to socially engineer a solution and probably pick a new tag, like #barcampblock) and of decay (that is, over time, as tags are used less frequently, other people can reuse them — no domain squatting!).