1. The design clearly answers "Who I am," "What I do," and/or "What can you (the visitor) do here."
If you're a well-known brand or company (i.e., Coca-Cola) you may be able to get away with not having to describe who you are and what you do; but the reality is, most businesses still need to answer these questions so that each visitor knows they are in the "right place."
Steven Krugg sums it up best in his best-selling book, Don't Make Me Think: If visitors can't identify what it is you do within seconds, they won't stick around long.
2. The design resonates with the target audience.
A homepage needs to be narrowly focused -- speaking to the right people in their language. The best homepages avoid "corporate gobbledygook," and eliminate the fluff.
3. The design communicates a compelling value proposition.
When a visitor arrives on your homepage, it needs to compel them to stick around. The homepage is the best place to nail your value proposition so that prospects choose to stay on your website and not navigate to your competitors'.
4. The design is optimized for multiple devices.
All the homepages listed here are highly usable, meaning they are easy to navigate and there aren't "flashy" objects that get in the way of browsing, such as flash banners, animations, pop-ups, or overly-complicated and unnecessary elements. Many are also mobile-optimized, which is an incredibly important must-have in today's mobile world.
5. The design includes calls-to-action (CTAs).
Every homepage listed here effectively uses primary and secondary calls-to-action to direct visitors to the next logical step. Examples include "Free Trial," "Schedule a Demo," "Buy Now," or "Learn More."
Remember, the goal of the homepage is to compel visitors to dig deeper into your website and move them further down the funnel. CTAs tell them what to do next so they don't get overwhelmed or lost. More importantly, CTAs turn your homepage into a sales or lead-generation engine, and not just brochure-wear.