I am thoroughly unimpressed by Marissa Mayer at this point. Yahoo is doing nothing but keeping up their 'trend' of buying too early or too late.
Tumblr while a great asset is going to be FAR to expensive to pay off for Yahoo at this point, and a lot of the junior assets they have purchased have no or little direction.
In my opinion Yahoo is on a quick slide towards 'AOL territory' where they have to slim down to their 'core' to make any money and will likely suffer innovation starvation until they are almost totally irrelevant.