I have had intermittent problems connecting to a router less than 6 feet away on my new MBA, whereas my iPhone and old 2007 MacBook worked fine.
When the issue was occurring, I noticed that only my network was showing up (although I couldn't connect to it), not any of the many nearby ones. When trying to connect to my network, I'd get an error message that said "Could not join "(network name)". A connection timeout occurred."
After trying the most common tips on the internet (creating a new location, renewing the DHCP lease, etc), nothing seemed to work. Then I unplugged my external USB 3 hard drive and the MacBook Air connected to the network immediately.
After searching a bit, I found an article that mentioned in some cases external USB 3 hard drives can cause interference with wifi. The drive was not only very close to the MBA, but directly in the line of sight between the router and the MBA. So I can still use the external drive, but I now have it located as far away from the computer as the cable will allow.
Problem solved! I wonder how many others with "MBA wifi issues" are having the same issue? Hopefully this helps!