With the recent additions of Pittsburgh and Syracuse to the ACC (and with UConn and Rutgers waiting in the wings) I thought I'd write up a quick and dirty list of a number of factors looked at from a BC perspective, each with its pro and con side. This is not meant to be an exhaustive list, so feel free to suggest others in the comments.
Pro: More schools located near BC. This will be beneficial not only for the teams that now don't have to travel as far, but should (theoretically, anyway) make it easier for fans to travel to some away games as well. Plus, while the ACC will likely always be dominated politically by the influence of the Carolina schools, at least the decision-making will be slightly more balanced from a geographical perspective.
Con: No real cons here from a BC perspective.
Pro: In recent years the ACC has seemingly been on the downturn in terms of the overall quality of its teams. While the new additions likely won't change much where football is concerned (although Pitt has fielded a few decent teams in the past), the basketball product will certainly be enhanced, and many are theorizing we may see the ACC tourney move to Madison Square Garden if the Big East dissolves.
Con: BC will likely fall further down the ranks of the conference, at least in basketball. Playing regularly against Pitt and Syracuse, in addition to other powerhouses like Duke and UNC, will obviously improve fan attendance and interest in the basketball team (and, as an offshoot, media coverage), but from a pure wins and losses standpoint this will hurt BC in the short run (although their strength of schedule numbers will be improved).
Pro: Adding new schools will expand the ACC's TV base into New York and Pennsylvania, and will make new TV and other distribution deals easier to make.
Con: Dilution of revenue for each school. With two new schools, any shared revenue will be that much smaller per school. Plus, I can't imagine Pitt and 'Cuse will be bringing in that much football money (which, as everyone knows, is the real cash cow for college sports). Still, I can't imagine the ACC would have agreed to bring the new guys in unless it thought it would at least break even.
Pro: By taking the initiative, the ACC has staved off dissolution while it looks like the hapless Big East is done for. Adding these schools not only ensures the ACC will remain in place but makes it more attractive to other schools, notably UConnn (ick) and Rutgers. In general the ACC will be in the driver' seat and will have the luxury of rejecting applications, as it apparently did today to West Virginia (due largely to issues with WVU's terrible academics).
Con: The only potential con here is that, by adding more northern schools, some southern schools getting offers from other conferences - namely, Florida State - may find other conferences more appealing, if, for example, the SEC should come calling. Obviously the ACC schools felt the same way, as they recently voted to raise exit fees by a few million dollars (the vote was unanimous, but I'm thinking that was largely due to FSU and maybe others not wanting to signal anything negative to the rest of the conference).
All in all I'd say the pros outweigh the cons, and the ACC voters, who obviously possess much more information, clearly agreed. We'll have to wait and see how this all plays out before making any judgments however - the big picture may still be years in forming. As of now anyway, it seems like there might end up being four mega-conferences roughly dividing up the country. The ACC, by taking the initiative, ensured it will be one of those four (with the others being the SEC in the south, the Big "Ten" in the north, and the Pac-Whatever in the west).