In my opinion, trading away Paul Pierce and Kevin Garnett should be viewed as trading away two one-year rentals of quality players, not as trading away two Hall-of-Famers, if you want to examine the value of this trade.
Pierce has a year left and KG may retire after a year, so I think it is fair to treat them as potential one-year rentals. I would feel differently if I thought there was a chance they would be re-signed for a three-year contract.
There are many reported iterations of the trade, so I will go with Pierce, Garnett, and Terry for Gerald Wallace, Kris Humphries, Keith Bogans, Marshon Brooks, Kris Joseph, three firsts, and the right to swap 2017 picks.
So, imagining this as an aggregate of three trades, you have Garnett for Humphries, Brooks, and a first-round pick plus Pierce for one year of Wallace, Kris Joseph, and a first-round pick, and every else in another trade.
For one-year rentals, a young player with upside, a first-round pick, and matching salary (preferably expiring) as trade ballast is a magnificent return. So, I consider the return for Garnett and Pierce to be fair. Sure, I want more, but I don't really expect more than that and I can imagine getting less in return. No one is giving up a likely 2014 lottery pick or a young player on his rookie-scale contract who has reached being good enough to be a solid starter instead of being all about potential.
That leaves Terry for the other two seasons of Wallace (time-shifted a year in the future), Bogans, a first-round pick, and the right to swap 2017 picks. That also looks to me like a fair deal. (The salaries don't match, but we're only looking at relative values here.)
So, overall, I think the value in this trade is reasonable. Arguably, when broken down into three trades like that, the Celtics win all three. By artificial separating the trade like this, I hope it helps you understand what is going on.