Stolen (and slightly modified) from Daily Kos:

The provisions of the Affordable Care Act that go into effect tomorrow, six months after enactment of the law: Ban on discriminating against children with preexisting conditions: as of tomorrow, insurance companies can't deny coverage to children under age 19 for a pre-existing condition. The ban will go into effect for adults in 2014. Ban on rescission: insurers will be prohibited from dropping a customer when they get sick or to search for errors in customers' applications to use as a basis for rescinding coverage or denying payment for services. Ban on limiting coverage, lifetime caps: Insurers will no longer be able to impose lifetime dollar limits on benefits--particularly hospital stays or expensive treatments for chronic diseases, cancer, etc. By 2014, they will phase out annual caps. Ban on limiting doctor choice in new plans: insurers will have to allow primary care physician status for OB/GYNs and pediatricians so that patients don't have to get pre-authorization or referrals to see these providers. Ban on restrictions on emergency services: insurers will have to cover all emergency care, in or out of network. Guaranteed right to appeal insurer decisions to independent third party in new plans; Young adults can stay on their parents' plans til 26 unless they have access to coverage in their workplace; New plans will cover preventive care with no customer costs--well-baby, mammograms, colonoscopies, etc. will be covered with no co-pays or deductibles. Many of these changes will not apply to existing plans. Health plans that existed on March 23, 2010 are "grandfathered," meaning they are exempt from implementing these new provisions. So free preventive care, unrestricted provider choice, level charges for emergency care, and the right of appeal do not apply to existing plans. Additionally, the coverage for adult children on family plans can only be extended if an employer plan is not available to them.