"Apart from a few patents listed, we hold a large arsenal of patents that we won't rule out using offensively against both developers AND users of open OR closed source software."
As a software developer and user, I'm not entirely comforted....
"Over time, we intend to expand the set of Google’s patents covered by the pledge to other technologies."
Now that's not a specific promise, so you can argue how much it should count for. But if you take it at face value, it seems reasonable to expect that they will be adding to the list.
The newer patents on tech like glass and machine learning are what are particularly concerning for todays startups, many of who are venturing into these areas, and these are nowhere to be seen in this agreement. The recent patents also have the longest shelf life and the most potential for nipping competitors "in the bud".
Google is becoming an entrenched company, they will eventually (or sooner) go from an innovative market position to a defensive one, and thats when patents start to look really juicy to the bean counters.
Also, if you're a developer working for a large google competitor, you may also have reason to fear at least for your job (which is quite personal).