"Mid-core" is a likely very overused tagphrase now that's become nebulous and almost lost any concrete meaning. From playing both this game and PvP MMOs, and having dealt with statics and players from both kinds of games though some behaviorial patterns emerged that stood out to me.
I would contend that in this game one would see a lot of "closet" hardcore players, that is players calling themselves "mid-core" or whatever other catchphrase, but they truly aspire to and act just like a "hardcore" player. I think once you really think about it you'll be able to come up with a lot of examples in your head: the player who uses a lot of slang, complains a lot about "X is bad, these Y players are sh***rs, don't you hate it when...," the liberal use of slang insults like "autism" and "re***d," etc. On a less vocal level, at least in this game, a lot of passive-aggressive behavior in VOIPs in raids rather than simply calling someone out for perceived poor performance.
To me the less vocal behavior stands out because in PvP MMOs in particular, people have no problems directly calling someone out, insta-guild kicking, and other harsh in-your-face treatment. Here, you don't see that nearly as often because "hardcore" has quite a negative connotation. Hence if you openly act like what people perceive a hardcore player to be, that label is going to stick. Anecdotally and "on-the-street" in FF14 I would contend you can tell that there's a negative stigma generally associated with "hardcore" players, and some of it honestly deserved even though those players produce the results to back it up.
In PvP games, results practically mean everything and the tolerance of the player base is generally high. Consequently, hardcore players are viewed more highly and such behavior is more acceptable. Here, in a mostly PvE game that tries not to pander to base competitive instincts and seeks to be more inclusive, the stereotypes of hardcore players tend to stick.
In sum, when you really think about it, there's a lot of so-called "mid core" and even "casual" players that are really just wanna-be hardcore players or just afraid of being open about it because of the stigma. It is an interesting dynamic for sure but also makes it harder on the community at-large, because as they say like-is-attracted-to-like, and when people aren't vocal it's harder to get a read on them in advance, which leads to bad chemistry all over the place.