Another way to look at this: They raised the bar by showing more of what a game could be.
They're professional voice actors. However, the majority of them had
no D&D experience before that first campaign.
Matt Mercer is an exceptional DM. Not perfect (nobody is), but the best I've seen in 40+ years.
Taliesin and Liam had a bit, Marisha a tad, but the rest were brand new. What thy bring is a willingness to put themselves out there and acting talents.
D&D is an RPG. A role playing game. Characters play a role in a story. The better the story and storytelling, the better the game. We can either say, "Critical Role is not how
I play D&D", or we can challenge ourselves to try the things we see in Critical Role that we are not doing and see if they make things even better.
A
lot of people are unwilling to try. Some are afraid to try more acting for fear their peers will laugh at them. Others do not know how to begin. Other people just think they do not like it for various reasons. However, many of those people (but certainly not all) will find that they have a very positive experience if they try.
And, honestly, if you read the DMG, if you look at the advice there, if you really think about what it says - Mercer and Critical Role are playing the game as recommended. There is a lot of hedging language in the books about how every type of game is fine and there is no wrong way to play - but then if you look at the meat of the advice in the DMG's style of play, the flavor of fantasy, creating a campaign, creating adventures, between adventures, etc... they encourage you to build the story around the PCs - which is the core of the way Matt DMs.
Green Eggs and Ham it, folks.
And if you want to call me a Mercer fanboy - Hell yeah, I'm a huge Fan Man. As I said - best DM I've seen in 40+ years. I've stolen a lot from him, including how to help clueless players without insulting them, how to add drama, how to use silence as a tool, etc...
Mercer had a fully developed campaign world to start??
Personally I start small with a big idea for the endgame but slowly build on things. I don’t have a word developed but rather one is created through the story.
Matt created the world over time - starting with one location and expanding it until it was his broader world. That is the 5E DMG advice.
He created it as he needed it rather than creating it all and dropping PCs into it. Both approaches have merit. I like to build the broad strokes first and then fill in the details as I go (which is what I think Mercer actually did as well).
I do not wan to be caught unprepared if the PCs travel to a neighboring country so that I do not need to make cities on the fly, but I want the flexibility to shape around the ideas of the PCs as they encounter the town - and there will always be a bit of fly creation.