I have my own top 5 (though similar in spots):
1) Rondo is better than he was in 2008, and much better at countering a defense that puts a SG/SF (Kobe, Lebron, etc) on him. On the flip side, Derek Fisher is worse than he was in 2008, or at least slower.
2) While the media has said that the Lakers are better this year than in '08, and all they cite for the Celtics is that they are older (and therefore are implying worse), they are ignoring one enormous fact: In '08 it took the Celtics 7 games to beat the Hawks and 7 games to beat the Cavs. This year the Celtics beat a much better Cavs team in only 6 games. Beating a much better team in less games = The Celtics are definitively better this year than in '08
3) The Celtics offense, especially Ray Allen, has been much more consistent and flexible this playoff run. Outside of part of the "comeback" game against the Lakers in '08, and the blowout game 6, Ray was cold. The Celtics offense also stagnated. This year the Celtics are more balanced, more efficient, and attack where there are mismatches.
4) Combination of Kobe showing up too much and the Laker bench (mostly Odom) disappearing. If the Lakers find themselves down by double digits, Kobe will attempt to take over. Whether he hits shots or not, the Lakers offense will eventually struggle and his teammates will stop doing the necessary things to win. I have seen this consistently happen with the same result.
5) Composure. The Celtics can perform on the road, in the dark, in outerspace, in complete silence, while listening to Celine Dion on max volume, or any other adverse condition. If the Celtics get down by 10 points, will they panic? Doubt it. If the Lakers get down by 10 points, will they panic? Kobe might. If you were a Laker fan, do you really trust Ron Artest to not hurt your team at some point during a physical, intense, and loud finals series? I wouldn't.