As with everything, it depends on the concept and the writing. Doctor Who got along fine on 25-minute episodes for 26 years (albeit it was a serial so each actual storyline involved anywhere from 2 to 14 such episodes). But there have been other half-hour shows in the past that have used the length quite well. The first season of Danger Man (aka Secret Agent), the original Dragnet, the original Twilight Zone, Honey West, Adam-12, the school drama Room 222 -- all of these shows could have gone to an hour but remained at 30 minutes and managed to get a lot accomplished and deliver some great stories. In fact Twilight Zone did so well as a 30-minute show that when the network ordered it to be expanded to 60 minutes it failed so badly that CBS had to pull a Leno and reverse the decision.
Of course, we're talking of back in the days when a half hour show actually delivered about 25 minutes of storytelling time. These days you're lucky to get 20 minutes, which is why a show like Entourage can get away with things in a cable format, but on mainstream commercial TV only sitcoms are able to use the form now. Which is a shame, because I'd love to see a kick-ass detective show or spy show come along (a serious one, not a comedy) that fills up a half-hour time slot.
Alex