Yglesias is still going on about how important it is to select someone for Kennedy's seat who is young enough to accrue the experience needed to become Chair of a Senate Committee and then have some real clout. As I said yesterday, that's just not the way the Senate works; Committee Chairs just aren't all that powerful. It's also worth noting that while there is a second path to formal position in the Senate, and that one takes very little time: Ted Kennedy himself became Democratic Whip in 1969, during his first full term. That's not unusual. It took both George Mitchell a bit under nine years to become Democratic Leader, while Tom Daschle took eight year years. And while chairing a committee does take some time, chairing a subcommittee happens very quickly. The main idea here, however, is that Senators do not need formal positions to be powerful; the nature of the Senate is that any Senator can be very important is she's good at what she does. You want someone who can become a good Senator, and of course liberals should want someone pretty liberal to be a Senator from a safe Democratic state.
I'd add another thing I'd look for in a Senator from MA: Democrats should look for someone with national potential. Democrats from Massachusetts seriously contested for the Democratic nomination for President in 1960, 1968 (sort of), 1980, 1988, 1992, and 2004, which would be six of the nine open nomination battles over the last half-century (five for nine if you don't want to count Bobby Kennedy). That's all four elected Democratic Senators during the period, the only Democratic Governor to serve more than four years, and one overflow Senator. Together they captured three nominations.
In other words, the voters of Massachusetts may well be selecting a future Presidential candidate, so I'd be looking for whatever qualities one looks for in a president, rather than worrying about who is going to chair the Senate Armed Services Committee twenty-five years from now.