The only difference I see is the way they assign numbers to new versions. Browsers and other applications could be updated quarterly or sooner for years before Chrome started doing it, and we loved it because it meant the software was actively developed, but the software author(s) would (rightly) consider it a minor version upgrade and just go from e.g. 3.6 to 3.7.
Even with Firefox, going from 3.6 to 3.7 could break some of your add-ons, so, again, I don't see any difference between going from 3.6 to 3.7 or from 3.6 to 4.0 or from 4.0 to 5.0 with some add-on compatibility problems. It's all the same to me.
So, to reiterate: As far as I can tell, the only difference I see is that they changed the way the numbers change when they update the browser. Though, to be clear, perhaps I should say that I do think that 3.6 to 4.0 was indeed a major update.