Interestingly enough, and possibly a first for the internet, I'm coming round to the thinking that EC and Maruco might be right.
I was taken by EC's stats, but wasn't convinced by the arbitrary choice of 11st5 as the weight cut off - largely because the "stat" that was always bandied about prior to the change was that you couldn't win the National carrying 11st or more.
So I did a bit of research. The following is the percentage of wins achieved carrying 11st or more in a variety of long distance jumps races.
Scottish National 4m1f 16%
Midlands National 4m1f 18%
Eider Chase 4m 45%
Welsh National 3m5f 24%
Whitbread 3m5f 33%
Grand National Trial 3m4f 31%
Grand National pre-op 4m4f 16%
Grand National post-op 4m4f 50%
What do I take from that? It's harder to win carrying a large weight the further you go, either by distance or by heavy ground. (I'm putting the Eider down to the class of horses - the top weights there would be carrying 10st something in the GN).
And the weights adjustment has clearly had too great an effect on the results of the Grand National.