Level AA is a little more significant, and makes sites accessible to people with a wider range of disabilities, including the most common barriers to use. It won't impact the look and feel of the site as much as Level AAA compliance, though it does include guidance on color contrast and error identification. Most businesses should be aiming for Level AA conformity, and it appears to reflect the level of accessibility the DOJ expects. 
I am struggling with this right now. Alt text, captions, audio (read to me) options are good. I work with an accounting software app that redesigned its palette to red, black, and gray. No green to signify income. Made no sense to me and their gray text (also very skinny sans serif) made reading many nav bars difficult. Some URLs are accessible from the build-out, but the DIY sites often don't have the same capacity. As with secure sites, I think google will be downgrading those sites with little or no compliance or accessibility add-ons.
Legal precedent is changing, and ADA compliance related lawsuits are becoming more successful, and the courts are seeing more of them as a result. Title III of the Americans with Disabilities Act pertains to private sector businesses. Lately, those protections are more frequently expanding into digital territory as web and mobile applications become more necessary in our day-to-day lives.
While legal considerations might be your biggest worry, making your site more accessible is simply good customer service. More than 39 million Americans are blind and another 246 million have "low vision," Another one million are deaf in the U.S. Add to that people with mobility issues that prevent them from using their hands and that's a huge portion of the country's buying power.
The text in the ADA did not originally mention websites since this technology was not widely used in 1990. But now that most businesses have a website, they need to make sure it’s accessible to everyone. Since we’re past the ruling date, all updated pages on your website are required to be at least grade A complaint, with grade AAA being the highest.

As I mentioned above, under each WCAG 2.1 principle is a list of guidelines, and under each guideline are compliance standards, with techniques and failure examples at each level. Some guidelines include only Level A items; others include items for multiple levels of conformance, building from A to AAA. At each stage, you can easily see what more you would need to do to reach Level AA or AAA. In this way, many websites include elements at multiple levels of accessibility.
Federal law isn't the only consideration for businesses. Additionally, each state interprets the law differently. Consider the case against Netflix in 2012. Lawsuits were brought in federal court in Massachusetts and California. Netflix was accused of violating the ADA by not offering "closed captioning" options for its Internet streamed movies. Illustrating the complexity of this issue, the courts reached completely opposite decisions. Massachusetts held that Netflix must comply with the ADA, while the California court found that Netflix did not fall under the ADA's definition of "public accommodation."

The ADA statute identifies who is a person with a disability, who has obligations under the ADA, general non-discrimination requirements and other basic obligations. It delegates fleshing out those obligations to federal agencies. The agencies issue regulations and design standards. The regulations have the details on the rights of people with disabilities and responsibilities of employers, state and local governments, transportation providers, businesses and non-profit organizations. The design standards specify how many entrances need to be accessible, how many toilet rooms and the design for those elements.  To know what the ADA requires, you need to read the law, regulations and design standards.  
×