
A few thoughts.
While many news organizations have portrayed this as Gates firing Pace, that's really unfair. Pace has served in the Joint Chiefs for six years, four (two terms) as the Vice Chairman and two as the Chairman. Although a waiver to the three-term limit can be done with the stroke of a pen, the general would still have to be confirmed once again by the Senate.
Secretary Gates was (rightfully) concerned that a confirmation hearing in the Democrat-controlled Senate would be a circus of grandstanding, posturing and recriminations about the past rather than where we are headed. To spare General Pace (and no doubt the administration) that ordeal, he opted to select a new candidate for the position - Admiral Mullen.
Mullen has been the chief of naval operations for almost two years now, so might face some questions about current administration policies, but won't have to endure the WMD debate, the decision to invade, intelligence issues, etc. He technically has not been in the chain of command for operations in the Middle East (or anywhere else for that matter) - the title "Chief of Naval Operations" is a throwback to the days when the different Navy fleets conducted their own operations.

General Pace is the first Marine Corps officer to hold the position of chairman. Prior to Pace, there were eight Army officers, four Navy officers and four Air Force officers in the position. Given the world political situation and American strategic interests, the appointment of a Navy officer is a good choice. Oil moves on the world's oceans, we move our forces by sea, trade moves via the world's waterways, and the United States traditionally has been a maritime power.
The founding fathers recognized this when they framed the Constitution, addressing the fact that the new nation had no Navy - the Continental Navy extant at the time of the Revolutionary War had disappeared. Article I of the Constitution includes this passage: "The Congress shall have power...to provide and maintain a navy....
Britannia no longer rules the waves, nor do we aspire to. That said, when there is a crisis somewhere in the world and Americans or American interests are at risk, the first question asked is, "Where are the carriers?" Having the officer responsible for training and equipping the United States Navy for the past two years is not a bad idea.