There seems to be a massive anti-Yahoo struggle going on in HN at the moment but I feel we need to look at this impartially. For better or for worse, the fate of the company is tied directly to the actions of Marissa Mayer now, this is no longer 1999 and Yahoo is no longer the same company. They have this image (which they rightly deserve) of being a big, bumbling ancient Behemoth of a company which provides no real…
I fail to see the connection between the acquisition of Tumblr and your vision of Yahoo suddenly becoming able to create new and exciting applications on its own as a consequence.
According to Wikipedia, Yahoo's main source of revenue is search and display advertising (by far). By purchasing web sites without any business model and trying to monetise their traffic, they remove value from these sites (for the user) and add nothing that these sites couldn't have done by themselves, in exchange for huge amounts of cash that could instead be spent on developing those "new and exciting applications".
They might get some value out of the talent they are acquiring with Tumblr on the long run, but I doubt that it'll be worth that much.