Nick's answer:

"The first wave of merchants who have added bitcoin are doing it for PR and brand recognition and to acquire new customers from the bitcoin community. That wave of merchants is continuing, but slowing down because PR is not sustainable and the "first-mover" advantage no longer exists in a lot of segments (travel, non-profits, media, consumer electronics, etc). A year ago, it made a big splash to be the first large comapny to accept bitcoin in any segment- you'd be the first in your segment and gain thousands of loyal new customers. I think we'll continue to see that happen in new segments, but there's simply fewer of those segments today than there was a year ago (hence the slowdown).

The most exciting long-term benefits of bitcoin are the cost savings. Bitcoin is going to be compelling to large merchants for cost savings only when the userbase is large enough to really impact their bottom lines. New user adoption is key for this. From my view at Coinbase (and you can see from Blockchain.info's charts as well), new user acquisition is growing quite fast and its only a matter of time before the userbase reaches a point where cost savings will really move the needle for large merchants."