I suggest that you go back and watch Trump's draft of that paladin deck again here:
http://www.twitch.tv/trumpsc/v/21604621?t=3h56m5s He talks a lot about his choices.
In this particular deck, he drafted 3 Seal of Champions (a buff card that gives the target divine shield and +3 attack), so he wanted to have a lot of 2 drops so that he was more likely to have a target for Seal of Champions on turn 3.
In general, in the first part of the draft, Trump just picks the strongest card offered. Then later on, he'll start to take the curve and other synergies into account.