I should say that I agree with TheExceeder right there. However, if I am to draw a line between the old days and the new days, the old days would be before Synchro. The playstyle differs greatly between the GX and 5D's era.
Though to be a
Honest, I myself split the "days" by the series/prominent theme (The Old Days, the GX Days, the Synchro Days, the Zexal Days, and the Arc-V Days respectively)
I hardly compare the "daylines" though, each of them has their own pluses and minuses;
The Old Days (Before GX):
(+) Peaceful gameplay (Outside the tourney, at least)
(+) NOSTALGIA FACTOR is too stronk! XD
(-) Very, very little variation
(-) Game gets shallow pretty quick, should find an innovation immediately
The GX Days:
(+) A bit faster gameplay than the previous Days
(+) Mill Deck is still something
(-) Still very little variation
(-) Unbalanced support for summon types (Why hello there, Rituals...)
The Synchro Days (5D's era):
(+) Innovative, groundbreaking aspect of the game is introduced that would change the game forever
(+) Fast-paced games that are still fun to play
(-) Mill Decks are not so relevant anymore
(-) Too much focus on Synchro itself
The Zexal Days:
(+) Introduced some cool cards, and the Xyz Mechanic
(+) Finally, some more variation
(-) Speed too fast to be taken if just starting
(-) Very unbalanced support in Summoning types (Why
hello there, Rituals...)
The Arc-V Days:
Can't judge too much because it's still just started recently
(+) Rainbow-colored variations~
(+) Vibrant mechanics and reviving of the card game as a whole
(-) Power creep is very visible
(-) Speed of light if playing tourneys
Link is the new Synchro, because it changes the definition of "Classic" in Yu-Gi-Oh!