Personally, I prefer the later seasons because I find they are darker and more intricate than the first three.
There's actually not that much action, explosions etc. in later seasons as some people seem to think. Yes, you have some episodes that are heavy on action, but even in episodes like "Sacrifice of Angels" most of the run time is devoted to character development and telling a story.
One of the things I noticed about later season episodes was that they just kept getting better at handling several storylines at a time. When the show was at it's best, it would effortlessly juggle three or four storylines without breaking a sweat, and it was sheer pleasure to watch.
IMHO the earlier seasons just weren't adept at that. In general, I felt they were in fact often more simplistic in that they would use the common TNG A/B story formula in which those two plots often wouldn't even connect. Later on, the storylines would overlap more often (I'm not saying they never did A/B later on, they did but not quite as often). This would contribute to greater complexity and allow them explore more difficult subjects and deal with the characters in greater depth.
There's many more things IMHO that put the later seasons head and shoulders above the earlier ones. But rest assured, it's not 'splosions and stuff like that
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First they were told they had to recapture the station in one episode, then they were told they could have four, but when they began writing they realised they needed five episodes and eventually they made six.
