Getting a big name guest star is nice and all but unless it's a good story it's a waste. An unknown actor can make a good story but even well know actors can't do much if the materal isn't there. The smacks more of a marketing ploy with the help of BBCA.
Unless you've read the script already, how do know it won't be a good story? And what is exactly wrong with casting a well known actor who will bring people who otherwise ignore Doctor Who into the fold? And to anyone who says Browder isn't a big name, clearly isn't aware of Farscape and Stargate's popularity, and yes I know he was a late addition (as was Claudia Black) but he was more than a worthy successor for Richard Dean Anderson.
They could have cast Bruce Campbell and you'd still have people saying "yeah so" except for the millions who will go "oh yeah" just as you've got a lot of people quite happy to see Browder's name associated with Doctor Who. And remember there's still the question of the next companion - no one's saying it has to be a female in her 20s. A guy in his early 40s who is world famous for co-starring in Farscape could fit the bill quite nicely (though I'm assuming this will be a one-off, of course).
Alex