It's more than just the generation gap. I rarely watched Star Trek growing up, and when I tried to get into it, started with the original series. It was ok, and entertaining to varying degrees. TNG did pretty much everything better: better writing, better development, dealt with deeper issues (torture, witch hunts, what is life, what is "reality," etc.), and had more in-depth characters. Picard was much more complex than Kirk, and of course it didn't hurt that Patrick Stewart is an incredibly complex actor.