Left-winging it on the airwaves: I dont' want to belabor my working thesis of "Conservatives: evil but entertaining; Liberals: good but shrill" but since you keep bringing it up. What makes Rush et al. entertaining is that they pound their desks and say how empty-headed the Democrats are. It's fun saying people are stupid - this is after all, the beacon of much comedy. The Democrats can't easily say how "stupid" the Conservatives are; after all, many of their ideas are academically sound. No, the Dems harp on how "heartless" the Conservatives are, and that the "little guy" or "Middle America" is getting the squeeze at the benefit of the Rich. This is anger, fuming anger. Anger has a short shelf-life (See Dean, Howard).
What sells (on talk radio) is humor mixed with intelligent rantings, or sex. Since the Dems can't say much on sex (see Clinton..oh forget it), they have to be funny and smart. Al Franken is funny and smart, but rarely for a whole book's-worth, and I whole-heartedly doubt for every weekday. What's left is whining, and it just gets old real quick - or it turns into prostelytizing (see, Garofolo, J.).
John Stewart, of the Daily Show (I know Eno, you get your news via the "wireless" and don't watch cable) is probably the funniest guy (with his writers) doing news-like commentary, but he's quite a-political. Harry Shearer (Simpsons, Spinal Tap, etc.) does an NPR gig on Sundays, and while his bits are funny, his delivery is deadly - and not in the sense that he's killing with laughter. George Carlin is perhaps the funniest of all Liberal ranters, but he's a bit long in the tooth - he'd probably do a good Andy Rooney segment, but not an entire show.