I thought dotless domains were coming, and put full support for dotless domains in SiteTruth.
There was a long discussion of this on the Mozilla developers mailing list. There are some dotless domains right now. A few country codes will resolve to an IP address, and one or two actually have a web site there. Try ac
A lot of software, some of it very low level, mishandles dotless domains. If you look up "ac" in DNS, you'll get a valid IP address. Browsers, though, usually try using it as a search keyword, or try it with ".com" suffixed. There was a concern that if every word typed into a browser's input box had to be checked for being a TLD domain name, it would overload the root servers and delay search responses. DNS TLD "no finds" are relatively expensive operations.
Down at the "getaddrinfo()" level, there's a known bug. There's an exploit for this that drives traffic to subdomains of "com.com", which is set up so that all subdomains of .com.com" are full of ad pages. Right now this is just annoying, but it could be exploited in more ways if single-component domain names became popular.
That's really hard to fix, because it's in the C library on most machines. Applications would have to be rebuilt.
If you put a "." at the end of a domain name, it's "rooted", and local lookups on your local network do not apply. Type "ac." into your browser's input box, and you'll get some domain registrar who bought the Ascension Island TLD.
ICANN actually did something right.