I think another thing that people forget when they talk about a16z is the fact that they seem to be quite hands on and they make a point out of turning technical founders into CEOs. Chris Dixon makes a point of the former fact in his blog post, and Ben Horowitz makes mention of the latter point in his interview with Kevin Rose on the last Foundation podcast.
Rather than 'picking winners' I think part of their success probably stems from the fact that they actually had a material effect in helping to create winners with something more than just providing capital - I wouldn't be surprised if some of their more dubious investments pivot into something else and find success. Definitely happened with their investment in Burbn which pivoted into Instagram, and Rap Genius has already announced that there are a number of directions they will probably move in.
Rather than 'picking winners' I think part of their success probably stems from the fact that they actually had a material effect in helping to create winners with something more than just providing capital - I wouldn't be surprised if some of their more dubious investments pivot into something else and find success. Definitely happened with their investment in Burbn which pivoted into Instagram, and Rap Genius has already announced that there are a number of directions they will probably move in.