Here’s a nice post by BBC writer James Cary on writing for radio. Some of his thoughts on career are specific to the BBC model, but his points on writing are true here in the States as well:
The script is king. Radio teaches you that in a hurry. There’s no hiding in radio – and so as a radio sitcom writer, you learn fast. If the show misfires, it’s unlikely to have been a technical fault. Most likely, it’s a script-error, a string or duff jokes, a confusing plot turn or a badly defined character. ie. your fault.
We’ve definitely found that radio allows telling, with a small-ish cast, and even just one or a couple producers (sometimes the same person who wrote the script), stories which would be much more expensive in any other medium.