One of the first things you will see on the field level concourse is an increase in Mets memorabilia. In the image above, there are large cards of past Mets. The Ebbets Clubs behind home plate were renamed the "Champions Club" as well. Lots of the unpainted concrete (especially in the stairwells) were painted orange and blue.
The center field large video board had some changes as well. Some of the uglier ads are gone and replaced by some more understated ads. Specifically, many readers mentioned in emails that they disliked the two large Arpielle ads to the left and right of the board. These were replaced by SHARP Electronics ads.
The bridge in right center field was renamed today. It was dedicated as the Shea Bridge. This was named after William Shea, the man tasked with bringing National League baseball back to New York after the departure of the Giants and Dodgers.
A big change is the lowering of the center field wall to an even height of eight feet. In the image above, any ball hit over the orange line is considered a homer. In 2009, balls needed to clear the high point of the wall for home runs. I don't think this will have much effect on the games. Only a few balls hit the wall in dead center in 09, so there won't be more than a few disputes due to it.
One more change is the changes in the concessions. Above, you can see a new "Box Frites" location. On on the opposite side is a Blue Smoke location. Also on the Promenade food court is a duplicate of large beer stand in the fanfest area. Additionally, there are many spots with beer on tap.
Be sure to check out all of the changes for yourself in 2010. If you can avoid some of the bad seats, Citi Field is a great place to see a game. I've been to 29 different parks, and by far Citi has the best food in the league.