Kesler was dealt for a player who outscored him last season, a bottom pair D, a 1st round pick and a 2nd round pick. I personally think Vancouver got the better of the deal there.
Iginla was a rental traded for a 1st round pick and two decently rated prospects. That's what rentals get you. Good deal for Calgary, really. As good as the Boston offer? No, I guess not.
Spezza was traded with one year left on his deal for someone I think will be a 20+ goal scorer every year, a decently rated prospect, a B prospect, and a 2nd. Dallas also gets a throw in player. For 80 point Spezza, Dallas wins the deal easily. For 60 point Spezza, I'd say it was pretty close to fair.
Notice that Iginla, Spezza and Heatley all had their values drop after being essentially traded away before nixing the deal. That definitely plays a factor.