I thought the episode was really good, and frankly better than I was expecting. The casting left me very disappointed, though. I wasn't expecting M*A*S*H*-level recasting, but damn they could have made some effort to change the character dynamic. Instead we get a diva to replace Mercedes, a season one Quinn kock-off, Puck 2.0 (literally), and New Rachel. I wouldn't be surprised if the New Directions suddenly add a feisty Hispanic girl, a bumbling oaf, and an Asian dance prodigy in the next few episodes.
Well their is some similarity, each o their characters are different, in some cases dramatically then who you say they are replacing.
Rachel and Marley have no character aspects that are similar. Just being able to sing doesn't make them the same. Marley is down to earth, in every thing we have seen so far, and not driven like Rachel was at all.
Puck and Jake. Puck was a bad ass, who with Glee learned to be a nicer guy. Jake is trying to be a bad ass, trying to be something (that he probably isn't), again not the same, in some ways quite opposite.
Wade, and Mercedes, Big Voices yes, but Wade will have completely different issues (though I don't really like the character, to over the top in my mind, but will give them a chance).