A lot depends on the quality of the story arc, there needs to be a coherent narrative to bring everything together, otherwise, yes, it's a never ending soap. Plus a willingness to break the ordinary (so, for instance, you get episodes like 'once more, with feeling' and 'hush' in Buffy). Otherwise there's potential for lot of bloat. So for instance, I liked Lost, but let's face it, there was enough material for one tight season, or basically two if they loosened the waistband a little. Six seasons was pure wobbly you-want-to-poke-it-and-be-disgusted flab. There was some fun admittedly navigating the plot holes. A medium I call uh?-space. Bonus bananas for the fact they were probably just making up the plot as they went along though. There's hope for my novel yet.
X-Files was a prime example of just not knowing when to stop. You did want to take it out the back and shoot it. I expect it would have thanked you for the mercy.
US show cancellations are brutal. Anyone sane would happily slowly chew off the limbs of the individuals responsible for cancelling Firefly for instance (not for Dollhouse though). The network monkeys are still trained to salivate copiously over the Nielsen ratings. The internet, DVD sales, that kind of thing is unknowably mysterious (it probably makes sense if you consider the Alex Baldwin character in the marvellous 30 Rock, who climbed the corporate ladder to be 'Vice President of East Coast Television and Microwave Oven Programming').
A special mention for 4400. This was slow, but OK. I picked up the entire box set for not much money, so I couldn't complain. Well, not till I got to the end of season 4, which finished teetering on the edge of a very high cliff – to find that it had been cancelled and their wouldn't ever be a fifth and final season). That's not a complete series, network monkeys. One day, when they least expect it, someone is going to get a limb chomped right off for this kind of the behaviour.
But no, I haven't seen Homeland.