No, OP, I disagree. I didn't find the mobs difficult as I leveled, nor do I find them difficult at endgame. You just have to work on your gear. Whenever I had trouble as I was leveling, it was clearly a sign that I needed to work on my gear a bit... I'd upgrade some mods/barrels/armoring, go back and kill those mobs with ease.
The poison pill is the story and the lack of end game content, as someone else said earlier. The story is great, don't get me wrong. However, it draws you in, you enjoy it immensely... but then you get to the endgame, and it just drops off from there. You run the same OPS, FPs and dailies many times over, press the spacebar many times because you've already seen the story and no longer care to watch it... you gear up... and then... not much. Had they spent as much time working on the endgame as they did the story, the game would be wonderful. Perhaps the endgame will improve over time. I really hope it does. Until it does, people will continue to leave the game... it's inevitable. People get bored, and go do something else.
Honorable mention: the game engine. It's a mess.