EDIT: Also another pitfall is that you can't compress all your JS into one big JS file saving time on requests too.
A (good) CDN speeds up content by having distribution points close to the client. An added benefit of a shared CDN (like this) is that resources can be shared by multiple pages, which will mean the cache will be shared by multiple pages. DNS lookup can be a factor in page load time, but parallelizable HTTP requests (especially to a location that is close by) can usually make up for it.
Google's recommendation is to use between 1 and 5 hosts per page: http://code.google.com/speed/page-speed/docs/rtt.html#Minimi.... For the type of clients this CDN is aimed at (people who don't have their own CDN) it is unlikely they have more than one or two hosts already used, so this CDN is likely to be useful for them.
HTTPS support is required for domains served over HTTPS. If you aren't using HTTPS then don't use it.
As the number of SPA-type "sites" grows, the ratio tilts towards JS and dynamic communications (XML, JSON, etc...). Though overall I don't think it matters much.
A more interesting (I think) potential effect, should a given CDN reach a significant hold of the overall serving market, is the likelihood of a cache hit for the relevant library.
It's a must have.
And so that, yunno, you don't compromise the entire page making the use of HTTPS completely worthless.
<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>
Source: http://html5boilerplate.com/
<link rel="dns-prefetch" href="//cdnjs.cloudflare.com">
Pingdom stats
Consider that the community CDN is compromised - if that file gets replaced with a different JS file, you've now provided an attacker an XSS hole into _every_ page using the CDN.
I have a reasonable trust in Google to secure their own servers against such a compromising attack, but have no similar reason to put faith in smaller companies/services.
for eg. I can see techcrunch and metric fuckton of google analytics cookies
separate this from the cloudfront clients or you have a potential security problem and a definite privacy breach
