Braves lineup was very lefty with Freeman, McCann, and Heyward all lefty bats (not to mention Chipper's strong side and Bourn--now gone). So, Upton balances Braves lineup a heap compared to recent seasons.
Also, keep in mind that Upton is going from an AL pitchers park to a neutral NL park. Upton's OPS+ over the last two seasons was 112, which is basically identical to Freeman and Prado in 2012. So, that's pretty solid offense for an up-the-middle position. A key is whether Upton's CF defense can be more consistent. He has good CF range but is a bit erratic at times. Big defensive shoes to fill compared to Bourn.
Upton at age 22 in 2007 had an OPS+ of 136. So, there may well be some upside left in him too, as he is still only 28 and is now getting out of a bad ballpark.