IMO, The Next Generation
took itself far too seriously for a show about people in brightly coloured pyjamas exploring the galaxy. The Original Series
had it's tongue planted just slightly in it's cheek, which made all the difference when something totally outrageous would come up. TNG fans point at something loopy happening in Trek and cry "WRONG!" whereas for those who enjoyed seeing Kirk take on Apollo himself, it's just another day in outer space...
(BTW, my first Trek was TNG. I just prefer TOS.)