There's really only one sanity test for a browser: whether it runs fast for you. A slow-loading, incompatible browser is one thing, but most modern browsers work fine for both Ford.com and Fark.com. The real issue is whether a browser loads fast for the sites you frequently visit.
It's a highly debatable topic, one that tends to be subjective and fraught with inconsistencies. For example, latency on the Internet can dramatically affect browser speed. One day, Google Chrome can load IGN.com faster than butter on a banana, the next day (say, when a new Gears of War 2 review posts), latency can slow the site to a crawl, and Chrome seems like a dud. That's why, when you see speed tests for browser that claim "Chrome loads faster" it's important to ask a few questions: loaded when, over what broadband speed, with what other apps running, on what machine?
Speed, of course, isn't everything. We ran into an interesting compatibility glitch or two completing some of our tests. If a browser can't finish a particular benchmark, it doesn't get a score.
What does it mean, ultimately? For the average user, it may not mean much. But for those of us who spend vast amounts of time on the Internet, for work and play, browser performance is a big issue. Waiting for sites to load is no fun when you're trying to get work done.