Blizzard Hints At New StarCraft, Launches Burning Crusade
For the most part, I wouldn't worry about that.
With the Burning Crusade, Blizzard has taken the approach that effectively all the new features are available to everyone, with the exception of the actual expansion content - the new zones, and the stuff found in them. The game mechanics, UI changes, class changes, talent changes - all of it is consistent between expansion and non-expansion owners.
Having BC means that you can level further and get better gear in zones and instances that non-BC folks don't have access to, but really, that's not terribly unexpected from an expansion.
Blizzard has also explicitly stated that their expansions are going to be major content injections; if BC is any indicator, they're serious about that. They're providing patches to everyone that do the "minor" and common updates, and are saving the big content upgrades for expansions. Makes plenty of sense to me.