Because there's so little ground in the middle in the Senate, it matters quite a bit whether Scott Brown will be joining the Mainers on the left of the Republican party, or if he'll wind up voting with the mainstream conservatives. If it's the former, then the current Senate will look a lot like the Senate from early in 2009. 59 Democrats (two of whom, the Benator and Holy Joe, present real problems with party loyalty), and then a pool of three moderates that Dems can try to pick off. If, on the other hand, Brown votes like Judd Gregg (or to the right of that), then the Democrats must keep all 59 and at least one of the Senators from Maine. In other words, two targets instead of three.
Tom Schaller lays out the possibilities here for Brown's future career. I think his categories are right: Brown could serve out his two-plus years and then lose; he could win reelection and have a longer Senate career (Schaller suggests Dorgan as a model, but I think a better comp for that would be Al D'Amato); or he could emerge as a national contender. In addition, this very useful post by Boris Shor places Brown just to the left of Olympia Snowe, based mainly on his votes in the Massachusetts legislature.
So, how will he vote?
If it's possibility #3 (try to get on a national ticket in 2012), then he needs to quickly establish a hard right voting record. In particular, as Shor points out, Brown is and has been pro-choice on abortion. If he wants to be on a national ticket, he needs to have a very quick conversion experience and immediately establish a strong pro-life record. Beyond that, it calls for a mainstream conservative voting record, far to the right of what his legislative record would predict.
If it's possibility #2 (try to have a Senate career), then Brown has the almost impossible task of keeping his voting record moderate enough to appear acceptable to the people who just voted for him yesterday, but conservative enough that his core supporters stick with him. Conservatives, eager to give Obama a black eye and get a 41st Republican in the Senate, were willing to overlook any signs of moderation, and Brown was able to oblige them by offering vague conservative rhetoric (it helped that he didn't have an opponent able to pin him down on anything). Now, he'll have a voting record. For a while, his giant-killer status will shield him from normal conservative wrath, but it's anyone' s guess if it's possible to walk that tightrope for long.
That leaves possibility #1, one-and-out. My guess is that this is where he ends up. He winds up around where Judd Gregg is: on the left edge of the mainstream conservatives. He won't be caucusing with Nelson, Lieberman, Snowe and Collins to form a five-Senator bloc to negotiate for "moderate" things; instead, he'll vote with the Republicans against 60 or 61 vote coalitions, but he'll sometimes join Democrats as the 63rd, 64th, or 65th vote. And then barring another fluke he'll go down to defeat in 2012, and if he's lucky and the Republicans win the White House, he'll get to be Secretary of One of the Many Departments Republicans Don't Care About. Or maybe he gets a show on MSNBC (he'd be too unreliable for Fox). In my reading of his campaign platform plus his voting record, that (Judd Gregg's voting record, not MSNBC) is probably where he's most comfortable.
As I said, though, it's really just a guess. What I can say for sure is that if he really wants to run for president (or VP) and actually have a chance, we'll see a flip on abortion very soon, most likely by April..