In fact, you may not believe it, but through his first three seasons, he's one of the best offensive second basemen ever. ("Ever" means since 1901. I'm defining "second baseman" as someone who's played 70% of his games at 2B. A player like Rogers Hornsby, who by the time he retired really wasn't a 2B, is not counted in this list, though he is counted below in the section about 2B with 2 straight years of an OPS+ above 108. Kelly's first season was as an LF, but he's still played more than 70% of his games at 2B over his career.)
Through the first 384 games of his career, all before the age of 27, he has an OPS+ of 108. Only 32 second basemen have ever done that in history. 10 of them are still active: Jeff Kent, Jose Vidro, Alfonso Soriano, Marcus Giles, Chase Utley, Dan Uggla, Dustin Pedroia, Ian Kinsler, Robinson Cano, and Kelly Johnson. 8 of the 22 retired players are in the Hall of Fame, and of the active players, Jeff Kent is definitely headed that way and at least half of the others have a serious shot, led by Utley.
And next year will be his age 27 season. He's entering his offensive peak. Kelly is one of only been 22 2B in history who had two straight seasons with a 108 OPS+ or greater before they turned 27. (9 of them are in the Hall, and a 10th is Pete Rose.) Other than Jose Vidro, who's basically finished, Kelly is the only active player in the group.
In their age 27 season, 4 of the 22 fell off offensively and posted an OPS+ under 100: Carlos Baerga, Edgardo Alfonzo, Marty McManus, and Snuffy Stirnweiss, all of whose careers as above-average offensive players were effectively over by their late 20s. 4 of the rest posted an OPS+ between 100 and 110, 4 posted one between 110 and 120, 4 between 120 and 140, and 4 posted an OPS+ between 150 and 190. (The final one, Bobby Doerr, did not play his age 27 season in 1945 because he was serving in the Army.)
In other words, an offensive breakout is quite likely, and even if he doesn't set a personal offensive high, he'll remain a very productive offensive player. He's still team-controlled for the next three years, and providing his production at a comparatively cheap price. Before we trade him, we should keep in mind exactly how valuable he's been, and just how valuable he's likely to be over the next few years. He's a very, very valuable commodity.