I was having a conversation about the show Heroes with Kenric the other day, focusing on their writers. It seems that they have the ability that many other shows' writers seem to lack -- the ability to satisfy viewers with substance, while getting us to keep coming back for more.
It seems like every episode has a cliffhanger, and in the next episode they resolve it. The same cannot be said about another popular (though becoming less and less) show, LOST. LOST is a show where the writers fail to tie up loose ends, proving to the viewers that even they don't know the answers to the many questions that left unanswered about this show. This proves to be a prevalent problem in many of these serial shows that need to provide something to keep the viewer from changing the channel.
Shows like LOST, Smallville, The 4400, they lack writers who know how (or don't bother) to bring the show forward, to make progress in the storyline. If motivation is what they're looking for, take a look at a show like Alias. Alias was an amazing show at one point in time, before they started to drag the story along, throw in re-runs and recaps, until the fans lost interest and they had to can it due to poor ratings. Yet in its inception it was an *extremely* popular show. LOST too, was highly successful when it came out, even winning an Emmy for best new show. Their ratings have dropped significantly but the writers have yet to clue in.
Smallville has less to worry about because it's a CW show, a channel that only kids watch (and a few of us pretenders). They have made some progress, but for the most part, it's 3 episodes of "meteor rock freaks" for every good episode involving Clark learning a new power, development of Lex vs Superman, and etc.
The 4400 is a lesser known show that I won't comment on because I don't know anyone besides me and my brother who watch it. The show has the potential to be as good as Heroes, but ends up being worse than Smallville.
Lately, Battlestar Galactica, a highly-acclaimed television show that I have been keeping up with, has fallen off the deep-end as well. Again, the common notion is that you need to sit through 3 bad episodes before you get a good one. Season 1 of Battlestar Galactica was incredible, absolutely amazing. But season two has left much to be desired.
The only serial dramas that I can think of that have had constant "forward progress", are 24, Prison Break and Heroes.