I think TFT is one of their most successful additions to the game, followed closely by URF. Of course, you are right when you say that TFT is not in it's best shape, but it is excusable considering how well it was received and how much traffic it had generated. Removing skill cap and just letting the game play be based on position and understanding the synergies and combos has giver everyone from every ELO a shot at reaching the limelight.
I like how you pointed out that the only reason for such a success of TFT is based on the existing success of LOL. There are numerous games as such in the app store, game store etc but do not gain much traction because they do not have such a crowd of such a magnitude.

The thing to look out will surely be if riot will be able to hold on to the explosive sudden success. They will have to keep adding new features each update to keep it competitive and exciting. Just like they did with TF and the new item drops.
Awesome post!