Microsoft has gone back and forth between treating SPs as feature packs.
With Windows XP, service packs were significant upgrades; XP SP1 added tablet PC functionality, SP2 was practically a new version of Windows.which was fairly major; XP3 had .NET features brought in from Vista as well as various security features (admittedly for business in most cases) rather than simply rollups and hotfixes.
Even Vista SP1 was a major update, improving power management, network speed, search speed and adding hardware support like Blu-ray burning to bring it up to the same performance as XP SP2 (according to Microsoft - it arrived just as Windows 7 was in beta so it never had a chance: I installed it on the way to the airport the day it came out, tested it and went right back to the beta of 7).
But although it included significant updates for business users using remote connections to a work server, for most people Windows 7 Service Pack 1 was little more than a rollup of updates and hotfixes (although it actually adds significant features for businesses using certain kinds of virtualisation from Windows Server). That's what Microsoft has been aiming at for years; service packs that don't add new features and that you already have if you've been accepting automatic updates.
The general popularity of Windows 7 makes that easier to achieve. So what does that mean for Windows 7 SP2 and when will we see it?