*Features: HTTP2 support... Free custom SSL (let'sencrypt support)... Wordpress integration plugin... Prepay system allows control over spend.
*Security: Two-factor authentication... hotlink protection.
*Documentation: Good documentation as well as regular newsletters with tips and tricks about how to implement CDN features as well as articles of use to website administrators. Very helpful knowledge base with a good getting started guide for those new to CDNs.
*Price: I've found this to be really great value over the past year compared to other CDN providers that I've researched.
*Payment options: Ability to pay with Paypal.
*Dashboard: Clean, clear information provided in the dashboard (usage etc).
*Support: The few times I've needed to contact the support team, they've been quick to respond with thoughtful and helpful answers.
The 5 zone limit (although I understand you can pay extra for extra zones), although the limit hasn't affected our site so far as we don't need more than 5. It is something to be aware of if you need more than 5.
Prepay credits expire after 1 year. Having an expiry is understandable, but for websites with little traffic it is good to be aware that you might not be able to use up the credit within the year. It would be great to have a system where credits rollover if you top up your account before expiry. Having said that, it's still good value with the minimum topup since you get all of the features that some other providers charge a lot for (custom SSL, etc).
Be sure to check out the knowledge base on the KeyCDN site if you are new to CDNs and need to learn about how the various features work. I found it very useful to learn about CDNs generally. Also, their newsletter is one of my favourites to read since they include information of relevance to website administrators, so be sure to subscribe to that too. Also, just be aware of the minimum topup amounts, credit expiry rules, and zone limits (as explained earlier) so you aren't surprised later on.
If you're comparing CDN providers, I found it helpful when I was comparing providers to check for what I consider 'hidden fees' for custom SSL certificates, and essential features such as HTTP2. For me, KeyCDN was an easy choice since I didn't need to pay extra for these (thanks let'sencrypt!).
The implementation of a CDN for our site was done to solve the problem of variable page-load speeds depending on where in the world the visitor was browsing from. Implementing KeyCDN has helped reduce the pageload time for the site generally. As well, I've since come to learn the other benefits of a KeyCDN, namely, reduced load on the origin server, usage analytics, hotlink protection, and so on.
The problem when looking to implement a CDN was that some of the other providers I looked at were charging exorbitant fees for something like a custom SSL certificate, or they didn't have features like HTTP2. KeyCDN has provided a really great benefit to our site in terms of performance, price, features, and support (including documentation and understanding the technical side to a CDN implementation).