I'm a strong believer that the strength of a team can be measured by the strength of the lines. If you have a really good D-line and O-line, it gives your skill players (pass rushers, cornerbacks, running backs, quarterbacks etc) more time and more room to do their thing.
Tom Brady didn't look so great last season but a big part of that was the fact that his O-line wasn't giving him 10 seconds to make every throw (like, say, Peyton Manning gets). Even Chad Henne looked great in a game against the Pats when his O-line was giving him all the time in the world.
Improving the lines should be the Pats' number 1 priority.