I've been in semi-large guilds for almost all of my MMO tenure. One of the advantages of large guilds is that whenever you log on, there's usually a critical mass of people online. Oh, not enough to raid, but usually enough so that you can start a 5-man, or there's some chat going on, etc.
Is this the case in 10-man guilds? Or do 10-man guilds normally have 5 or fewer people online at non-raid times? Or do 10-mans work best as small teams inside larger guilds?
It seems to me that this is one downside of the push towards smaller raid sizes. Larger raid sizes required larger guilds, making it more likely that one would log into a bustling community at any given time.