Selig placed interleague play into the game of baseball in 1997, giving teams who play in different leagues to get a chance to see what it'd be like to play teams outside their own league. This was great for baseball, as it drew more fans to the ballpark, because they want to see an "obscure" team play their own. When interleague was first introduced, teams only played outside their own league in the summer months.
This season, Selig made a modification to the traditional interleague play, as now there will be at least one interleague series on every single day of the season, even giving fans more opportunity to see their teams play against an "obscure" team.
Selig has put in a phenomenal amount of effort into "cleaning the game" and getting rid of players who have used PEDs. Being commissioner through the Steroid Era, Selig knows that he has to keep all the players stats and abilities safe, as he has called for more mandatory testing, and has given harsher suspensions. Selig has done a wonderful job at this.
Wild Cards (and other postseason changes):
Before Selig, Wild Cards were unheard of in the game of baseball, as there were two divisions, and the winners of both faced off for the pennant. Selig had the genius idea of adding a third division, and a Wild Card team, to be able to make the postseason two rounds, instead of formally one.
In 2012, Selig added a second Wild Card, giving the playoffs three rounds, and making the postseason more exciting. This gave other teams the chance to make the playoffs, although they may have not without the second Wild Card. This gave more teams the chance to make their games meaningful, and a chance to get into the postseason.
Also, Selig made the All Star Game count for home-field advantage in the World Series, which, to this day, is still debated on weather that's the way to go or not.
All in all, Selig has been one of the best commissioners ever. Even though he has only been the 9th commissioner in baseball history, he has done so much to be able to help the game keep its popularity, and keep its relevance among all the fans. The games became more exciting, more meaningful, and definitely a lot better with Selig in charge. At this point, it's unknown who will take over, but I think Selig will play a big part of that.