The best features are those that just work. They don't require special settings, answers to all kinds of questions, advanced interfaces and reading the manual to understand how they work.
Even if Google's search algorithms have evolved a lot in the last 10 years, Google works without having to type anything else than a query. It doesn't ask if you want personalized results, recent web pages or if your query has anything to do with celebrities. You can get away with spelling errors because Google automatically detects them, you can also type ambiguous queries without seeing a dialog that asks you to be more explicit. Now you don't even have to specify if you want images, news or videos because Google adds them to the list of search results.
The "add subscription" box from Google Reader is smart enough to take care of all the possible situations. You can enter a feed, but you can also enter the address of a site. Unlike My Yahoo, Google Reader detects if the site has feeds and picks the first one. But what if the user types New York Times? Google Reader shows the feeds that match this query and lets you choose the one you like. The feature could be improved by automatically subscribing to the top result for navigational queries like TechCrunch, where there's a single best result (at least in English).
Another feature that just works without human intervention is auto-save. You'll find it in Gmail, Blogger, Google Docs and it basically saves your text frequently so you don't lose what you type if your browser crashes or your Internet connection is down. You don't have to setup this option or mention how often you want to save your text.
But things aren't that great in Google Calendar, where you have to choose between 5 options if you want to add a calendar:
... or when you constantly need to choose between iGoogle and Google Reader when you subscribe to feeds, even if you only use one of the two products:
... or when iGoogle asks you location after you select a theme even if you've already added your location in Google Maps.
The features that just work are most of the times barely visible and that's a good thing. They're a part of a system that delivers what you want without constant interruptions and annoying workarounds.