I'd say LoTGD really is more of a fad type of game.
Think about it, there really is nothing to keep people coming back to play it over and over again.
Most get so bored they don't even beat the dragon once, let alone the 16x you could.
There's no real storyline, or side-quests. Definately no Co-op.
Might sound weird to say such about a php-based game, but it's those things that really keep people interested.
I think that what would make it better would be if it was planned out more; perhaps used Flash or Silverlight for a front-end for a bit of graphics and inter-connectivity with users online at that moment.
Even if that inter-connectivity was just real-time chatting.
Something that appealed me to it many moons ago was the social aspect of playing against you guys. It was the closest thing we all got to an MMO, or even an online game.