A lot of people see DQII as the great successor to DQI. Well, I can't share this opinion, but this is most likely, because I've played the first three DQ games back to back in a couple of days.

It feels like a weird in-between step, redoing and trying out a few things that also became standard later on, but it all doesn't feel that well made yet. Doing a direct comparison with DQIII is giving everything a beta-ish feel.
I blame that on the rushed development.
Otherwise, the game is not that different from the first part, but it is unpleasantly unbalanced.
Yes, Dragon Quest I is grindy. Dragon Quest II is grindy and unbalanced.