This is stupid on Google's part. People are used to mentioning others via "@" (unless, maybe, Twitter has copyrighted/patented this). Forcing people to use "+" will just make adoption of G+ harder. Google should remember that they're the underdog here, playing catchup to Twitter and FB. Raising the barrier to entry only hurts them.
people have been using @ to mention others since pretty much the beginnings of online discussion, way before facebook and twitter (I'm agreeing with you - this is a stupid move by Google and goes against 15+ years of convention)
I think that, while nerds are used to mentioning others via @, regular people are used to putting @ in email addresses. Frankly, I find this change frustrating (for the reason that " is not the opposite of -) but I don't believe regular people are going to have any more difficulty with +Adam than they did with @Adam.