If WAR's legacy is that it makes Blizz get its shit together and start making changes with their game, so be it.
There are fundamental mechanics in WAR that I wish Blizzard would implement into their engine, but Iczer said it best. I don't agree that there is no content at 40 but I believe they needed something a bit more considering when they released the game.
They should have waited and released the game well after WOTLK came out, because the timing played right into Blizzard's hand. I know in the current market it's not really economically feasible to just wait 6 more months but it would have helped a lot.
The population is the main reason I've switched back to WoW. Frankly, all my friends got fed up with getting zerged by 40 people when we had 15, and left. WoW is a much easier game to sit down and play with 5-6 buddies...in that sense it's even more casual than WAR.
But with all of this said, I've given WoW a lot of hate in the past, and honestly I'll be the first to say this expansion is a VAST fucking improvement over BC and even WAR. Mechanically I still fundamentally disagree with the CC of WoW, but overall this expansion is a great success.
The game is darker, the zones are much more detailed and epic (Icecrown is perhaps the most badass zone in any MMO I've played) deep arms in warrior got some love for PvP (DEEP arms, not MS.), some of the RNG is gone with the removal of mace stuns and the changing of windfury (now if we could get rid of blackout, imp hammie, imp wing clip, etc) and they are making an honest effort to integrate utility into the prot tree for PvP. The new BGs (the world BG and SotA) are more complex and objective based--a nice change of pace.
So I actually applaud Blizz for knowing what they needed to do to get some of the WAR players back--even some of the more fanboyish ones like myself. Again, if this is WAR's legacy, to give Blizz a bunch of reasons to improve their game, fine by me