I sort of agree with the statement above re: Megas. They're fine on some Pokemon, like the starters. Nothing additional was going to happen to Venusaur; giving it a special "Burst Mode" is fine by me (although Charizard going two sucks and is just ridiculous favouritism). I'm even okay with some two-stages - Mega-Steelix is fine, Steelix was always rather powerful and it's difficult to see what they could have done with a straight up evolution that wasn't just moving stats around rather than being an actual increase in power.
Some Megas, though, really just piss me of because that line definitely needed another proper evolution and didn't deserve to be shut down like that. The Megas of Kangaskhan, Pinsir, Heracross, Houndoom, Mawile, Medicham, Manectric, Banette, Absol, Abomasnow, Slowbro, Sableye, Sharpedo, Camerupt, Altaria, Glalie, Lopunny, and Audino all would have conceptually worked as just plain evolutions (with lower base stat totals, obviously), and not wasted the potential of these lines - especially considering about half of those Megas aren't even worth using in the first place, meaning that not only are some of these lines useless, they are now *permanently* useless because there is no chance of any extension to that line in the future.
Also, the Mega mechanic as a whole is just not that fun or interesting. You can work out almost immediately whether your opponent is using a Mega, either because a lot of Pokemon with Megas are not that good before Mega-evolving, or because of hold item effects, and for many of them it makes the entire flow of a battle concentrated around the respective Megas on either teams, diminishing the impact of everything else.
Honestly, it can't really be undone now, but it was one of the worst things to be done to the series for a while, to me. If the mechanic had to exist, I'd have preferred it if, I don't know, some Pokemon just automatically triggered to Megas if they hit 1/3rd HP as a kind of Break Limit, and the Pokemon that could do this were more or less restricted to the starter lines, the pseudos, and maybe a few additionals like the Alakazam and Gengar lines.
As it was, it was just given to too many Pokemon that felt like unfinished business.