Personally, I'm a fan of arc plots, always have been, so that's not an issue for me. But it's true that once you raise the stakes as high as they'll go, you have to do it every time or the show feels boring, because high stakes have become old hat. Buffy, I feel, suffered from this after the first three seasons, but I thought Buffy recovered by changing the stakes -- Seasons Five and Six made the stakes more personal, where the concern wasn't so much about the end of the world as about the people and connections that were threatened (Dawn and Buffy's sacrifice, Willow, Tara, etc.). Season Seven went back to big impersonal stakes and, I felt, couldn't pull it off as well.

I had the same problem with 24. After the nuke in Season Two, followed by the virus in Season Three, it felt like they seriously struggled to keep the threats from becoming redundant, old hat, and sometimes just a little bit ridiculous.

Thankfully, I've personally yet to feel let down by the fact that the new Doctor Who season isn't (yet) trying to raise the stakes above Season Four or The End of Time. As long as the characters and stories are good, I'm okay with arc plots, but I do agree that if they try to raise the stakes too far again (and again), it will become a problem.