For me the main differences are 1. Brand power 2. Culture and size
1. Brand power: this one is the obvious one, Kearney does not have the brand power of MBB, although it is well positionned right after the big three thanks to its long history as a consulting firm and its worldwide presence.
2. Culture and size: Kearney is still very much an "SMB" in its spirit and operations. Each office/unit is quite reasonable in terms of staff (from ~20 to 300 people) which results in a very friendly/family atmosphere in each office, where everyone actually knows everyone (and not just your desk as in other larger firms). One downside is that you have limited leverage of the network (e.g. knowledge management at firm level, office transfer etc.)
And to address the elephant in the room: the projects are the same as the MBB - Strategy, DD, ops, you name it.