It doesn't show different results depending on who's making the search, it doesn't track or record user information, it provides you with optmized results and it's built on and contributes to Open Source.
The engineer side of my brain immediately said, “Well, not doing per-user search results and not tracking users is EASIER to build! That's awesome that he's getting away with building less and tauting that as a good thing.”
People usually positively correlate the number of things you can click and look at with how “good” a product is. Microsoft traditionally has always had a problem with this, and a lot of other enterprise software does this as well, usually so they can just check off a feature request checkbox, with not considering how these features impact why people use the product in the first place.
One piece of software I use where they are proud of, or at least point a finger to, the features they don't have is Pinboard, where it has routinely called itself the “anti-social bookmarking service”, due to its lack of a full-fledged social network in the Facebook sense of the word.
The current feature set of Pinboard is exactly what I want. It keeps my bookmarks, lets me see a few friend's bookmarks with no automated discovery so there's not the add-everyone-you-ever-met problem like most other social networking sites, and then just gets out of the way. I don't need anything else and anymore things to click and look at would negatively impact the things I do need it for.
I'm sure these social features are easily in the realm of Maciej's technical ability, but developing a social anything, involves things likes privacy levels, explore of some sort, content and comment recommendations, and each of these has its own scaling problems, not just in hardware and software, but in community support and abuse prevention. I'd rather Maciej's time be spent making the current features better and attracting new users so he can make a good living and keep the service running.
Not having Feature X that every other product in your class has could absolutely be a defining feature. If a feature is not in your wheelhouse, for every user that finds it useful, there's at least one other user that finds it bothersome, or at least in the way of using your product's main focus.
I wish more of the services and products I used would be more strict about what they build and apply laser-like focus on that what their product is. I'd rather use many, small sharp tools than a few, large dull ones.