My biggest problem here, reading the document, is there is nothing these rules enable me to do in a game of Pathfinder, either as a player or as GM, that I cannot already do with the base rules.
The characters are not, ultimately, different than they were before when you tack "Mythic" on them. Instead of my group of heroes fighting a monster, it's now a group of Mythic Heroes fighting a Mythic monster.
I do not buy the arguement that tacking Mythic onto heroes makes them stand out more. Your characters uniqueness of strength is not a game mechanics issue. It is a setting issue. If a level 15 Paladin is not a legendary knight of virtue who's deeds have rarely been matched in all of history, it is because you have chosen to play in/write a world where level 15 paladins are common. You would have no less problem if you made Mythic ascension a common occurrence.
"So what. You ascended. Big deal. Get in line. Call me when you get to rank 8, at least"
I also do not buy that this system is needed for PCs to be known for Mythic deeds. There is nothing in the rules that says your PCs cannot perform great deeds that will be the topic of legend for ages to come. I think it is a bit presumptuous to refer to non-Mythic characters as simple sell-swords just going through the motions to get experience and money. Yes, this is a kind of game you can run. You can also run a game where your party slays evil beasts sent to ravage innocent towns by dark gods. And guess what. You don't need these rules to play that game.
Even the adventure IN THE PLAYTEST could be done without Mythic rules. Just have the god provide a boon in the form of whatever fitting buff you would like. Have the encounters be level appropriate. Take "Mythic" off the Hydra. The party still comes upon a town who, as a part of their festival accidentally calls down divine retribution and has a might adventure for which they are hailed as heroes, just without the number inflation.
The alternate reward system, based on "feats" rather than xp is interesting, but could be just as easily implemented without the rest of the Mythic rules as an alternate xp system. Ultimately though, awesome things happening in your game is a matter of setting and session writing, not game mechanics.
Ultimately, if you are going to offer me a set of mechanics that are supposed to make my PCs "exceptional" (assuming that they were not already), they need to be able to do things they could not before. As it stands they simply do the same things they did before, but with bigger numbers.