Firstly, a disclaimer. I am pro-BoA, but not 100%. I wouldn't mind seeing BoE or BoT or some other sort of hybrid approach introduced.
That said, there are 5 common misconceptions and oversights I see from the people protesting about it on the forums making.
I'm going to leave discussions over how BoA will kill Blackmarket sites alone for now, since this has been covered ad-nauseum before.
1) Not everything will be BoA.
Unmodified rares and the like will still be freely trade-able from what we know.
2) Rares will still be good.
While legendaries and sets will generally be better, from what we've seen from datamining there will likely be some awesome rares too, and more than likely a near perfect rare could end up being near BiS. Again, we know nothing for sure yet, but this seems to be a reasonable assumption at this point.
Especially post droprate nerf in the Beta, you are probably going to be quite happy to get an awesome rare.
3) You can still trade with your friends in the same game.
If an item drops in a game with a friend, you can trade it and help them out if they need it. Many of us are suggesting something similar being done with clans.
4) Account selling and botting won't become *that* widely spread.
At least nowhere near the extent that item selling is now. Sure, you will always have an extreme minority who will spend ages bypassing the system to do this, but there are a *lot* of safeguards to prevent account trading, and doing it runs a massive risk of getting ripped off.
Botting is another issue entirely. But one of the major reasons that botters are so common is due to how easy it is to sell gold and gear. If that stuff was all BoA, not only would there be less incentive to do it, there would also be a *lot* more risk.
5) Trading does affect others.
Far too many people seem to think that if unrestricted trading is allowed with loot drops balanced around self found, that people can just choose to opt out of it.
Thing is, as I've said in another thread...probably the most common 'goal' in D3 is to have a Powerful Character. And in games like this, power is relative. Trading will give a massive advantage, and thus for anyone wanting to play the game 'right'...there would be a serious incentive to trade.
And that's without PvP. Once PvP enters the arena (hah) then an entirely new can of worms is opened.
So yeah...I'm fine with BoA. I'm also fine with BoE or BoT. But overall I do feel that the quasi-BoA we seem to be getting now is going to be best for the game.