I was getting pretty tired of the Dominion War arc by S7, that's for sure. I was happy they were wrapping things up, but I wouldn't have been happy in S6.
A good example of the opposite problem - a show with too much needless complication, or complication that is dumped on us too soon - is Caprica
. That could have been very powerful as the story of two men fighting over the same "daughter" - one of them the real father, the other the creator of her virtual doppleganger - but the series was front-loaded with an indirect conflict between the protagonists that took forever to take shape, gangsters, religious terrorists, talk show hosts, polygamy, teenage angst - AUGH!
is doing it the right way so far. Start simple, drop a few very subtle foreshadowing hints along the way of future complications, and then reveal the complications only as needed, to keep things interesting and fresh.