I was intrigued with the possibility of him picking Tammy Duckworth, even though I knew she was more of a longshot than others were. I don't know quite as much about her as I do some of the rest, but I like her career (yes, it has a few warts, such as during her VA time, but then again, whose doesn't) and in general what she's done since she's been in office.
I do think that she would have helped Biden in terms of getting some military votes to come back from the right, something that any Democrat wants/needs.