So I've done a few sessions of Mythic so far and I must first say that I like the concept and feel of Mythic, but I've noticed a few things that I'd like to bring up.
Amazing Initiative: There is already a thread for this but I figured I'd put forth my solution here as well since it's part of my overall thoughts on the playtest.
You gain a bonus to your initiative equal to your mythic tier. In addition, you can spend one use of your mythic power to gain an extra standard action on your turn. At 6th Tier you can instead spend two uses of your mythic power to gain an extra full round Possibly not the best worded there but that's my thoughts at the moment.
Lesser Trials: These have been a headache for both players and GM. Too much meta-gaming and needless bookkeeping in out opinions. If these make it into the final product I can safely say I won't use them. The Mythic Tiers progressing basically as the plot demands seems the best way given the general feel I get from Mythic and my players seem to agree.
Flaws: This has been one of the biggest headaches for both me and my players. The Flaws are unnecessary in our opinions, they either don't come up or are needlessly debilitating. Had a player take the weapon weakness and chose scythe since he figured it wouldn't come up much, well as per Murphey's Law that was one of the random enemies I rolled for a battle and as one might expect he got seriously shredded by it. In summary, my players and I feel you don't need essentially a kryptonite to be a mythic hero. It's not he heroes weaknesses that make him noteworthy, it's the fact that he's better than you that does that.
Mythic Damage: Players really didn't like the extra bookkeeping when this came up, I think this needs to be fixed, or streamlined somehow.
One small gripe I have is the just how much the mythic characters stands above the non-mythic characters. I know that's the whole point of being mythic but the sheer number of abilities that give mythic characters bonuses when facing non-mythic foes kind of bothers me. Just a small personal gripe so nothing too wrong with the playtest in that regard.
So far everything else that was used has worked fine, though just from casual look overs some Mythic Spells and some of the path abilities could be abused as they are now. Will post further updates as more playtesting is done.
EDIT: If this is the wrong section to post this type of feedback feel free to move it. Put it here cause I wasn't sure and it's mainly GM thoughts and comments.