I know what you mean about the early seasons from those series being a bit listless. DS9 really came into it's own around season 4 imo. With 7 seasons for each TNG era series though, they're all bound to drag a bit. I wish they'd continued with Enterprise as well, I just hated Archer as a captain, and towards the end the show started getting ultra-fantastical. I dunno how well 7 seasons of Enterprise would have gone over heh.
The sets are repetitive in TOS, I guess I just love how darn colorful and 60s-esque the show was. I also enjoyed it as a look into 60s era world-views and how sci-fi proposed to deal with the then current issues. I think there was an episode that essentially equated to a justification for the Vietnam War lol.