I feel like most of these comments really do not capture the difference between retail and "classic." The largest difference is that "classic" (L75 era) is a very passive experience whereas retail is a much, much more active experience.
On "classic" the defining feature isn't so much the content but rather the obstacles between you and the content. You will spend a great deal of your time waiting: waiting for a party, waiting for a replacement for a party member, waiting for an airship, waiting for the airship to dock, walking to your camp, waiting for MP to regenerate between pulls while leveling, etc. Even some of the prevalent endgame is literally just waiting around for hours waiting for a Notorious Monster to pop. And even if your group is lucky enough to get the claim on the monster, there's a good chance you'll be sitting in the reserve party waiting for somebody to either die or run out of MP for you to take their place in the main battle party for a bit.
Retail, in contrast, is extremely active. We spend our time *doing* content. Constantly. Whether you have a large group, a small group, or just playing solo, there is always something you can be doing to improve your character. My typical game day consists of doing three different endgame events back-to-back-to-back with my linkshell for three to four hours. And when I'm not doing linkshell events, I can focus on leveling, crafting, Ambuscade, campaigns, reviewing gearsets, *playing the game!*
This isn't to say one is better than the other. But classic is much more laid back and passive whereas retail is much more active.
A secondary consideration is that retail is a much more complex game than "classic." Retail has many more jobs, many more spells and abilities, many more ways to improve your character, subjobs cap at L59 which unlocks more viable combinations, gear is more important, etc.. It's not uncommon for a single job playing at the highest level to utilize 80-100 pieces of different gear. Heck, my BLM has 3 different sets just for Magic Bursting.
Additionally, retail is still an actively developed game that has Notorious Monsters that have still not been bested by anybody. "Classic," however, is more or less a "solved" game with encounters that have been figured out for 15+ years.
As some have pointed out there are many differences between retail and "classic." Retail has solo-friendly aspects whereas "classic" tends not to. But this isn't to say retail doesn't demand partying if you want to tackle the most difficult content and progress your character as far as you can. Some have pointed out that classic is more "grindy" which isn't actually true. Retail is definitely the grindier of the two options. . . it's just that retail's grind is diverse whereas "classic" primarily is just leveling up. No, these aren't the real differences. The big difference is retail is active and "classic" is passive.