Responsive Web Design
In 2012, the amount of people surfing the web on a mobile device surpased those browsing on laptops and desktops combined.
Responsive Web Design (RWD) is a browser feature that premiered in 2009. It allows web developers to modify the design of their pages depending on the size of the visitor's screen; it also eliminated the need to create a separate website or a mobile app for proper rendering on handheld devices.
This was a big deal when it came out.
It is now expected of any modern site.
How hard is it to make a site responsive?
Technically speaking, it isn't that hard. If I were less of a developer and more of a businessman, I would have seized the wave of site updates as businesses transitioned into the age of mobile-friendly web design. Plenty of people did, for better or worse.
Some frameworks and pre-built templates claim to offer it automatically, but that should be taken with some skeptisism. Yes, they can work properly, but few of them do this automatically. Of the ones that do, it isn't always perfect.
Responsive web design is certainly not a selling point. You should be
wary of a designer or developer that promotes it like it's a huge deal.
It is, however, something there is no excuse not to use on every single webpage in your site.