First of all, because of the dimensions at TF, left field is more spacious IIRC, but right field is trickier due to the high wall, with an overhang and limestone.
It seemed to me that Arcia was more comfortable in right last year than he was in left in 2013. Further, I always saw Arcia as a right fielder. He's got a good arm, but limited range. However, he has played left field before and actually saw much more time in left than right with the Twins in 2013.
I see the move of Arcia to left as short term and in my opinion it made the Hunter acquisition much more problematic. Time will tell, I guess, but to me this just delays Arcia's progress in dealing with the quirks of Target Field's right field.