The player's introduction to Mega Evolution in Pokémon X & Y would have been easy to mess up, but I think that they got it right.
By the way, there are some spoilers in this post.
The way that we get a Lucario from Korrina still feels slightly forced, but at least they put in scenes prior to that that indicate that it was taking a liking to us. The main good thing about this is that we don't have to level up a pokémon to the Mega Evolution point on our own. From what I recall, we only get one (I'm not counting Mega Blaziken since that promotion will end), and I was trying to do a team of only new pokémon, so finding out that I had to swap out my Gogoat for my Bulbasaur and grind it until I got Venusaur would have been tedious.
Since they're including most pokémon from every generation in these games, I think that I'll eventually make a team from each generation. I just realized that I can include both Mega Venusaur and Mega Aerodactyl on my Gen I team - at least after I dig up some Old Amber.
I do hope one thing, going forward: If they decide in Generation VII to give a new Mega Evolution to a previous-generation pokémon, then they should at least not try to do any fancy tricks to explain it away. You just never had, I don't know, Dragoniteite or whatever. It's that simple.