Oh okay, you've been doing this for 20 years, so clearly you know what's good.
First and foremost, HTTPS protects your users. Posting a news update on a user forum may cost a dissident his life in an oppressive regime; A strict workplace may terminate employment based on an employee?s browsing activity; And of course, the Snowden affairs have clearly shown governments simply can?t get enough of this data. Using HTTPS makes it dramatically harder for these players to know what users are doing, and helps you maintain your most important responsibility ? your users trust.
Unencrypted content can easily be tampered. In addition, unencrypted pages are often in the path to secure ones. For instance, consider a shopping site where a product page is unencrypted, but the actual purchase flow uses HTTPS. A man-in-the-middle can change the unprotected product page, making the ?Add to Cart? button go to their evil copy of the website, and the browser (and user) will see no difference. If you only want your users to see the content you actually posted, and want their actions to always reach you, use HTTPS.
Roughly 18 years after its inception, HTTP/1.1 is finally getting refreshed. It?s successor, HTTP/2, has been officially completed in May (2015). HTTP/2 further evolves Google?s SPDY, and includes many significant improvements over HTTP/1.1, ranging from request multiplexing to header compression to server-side push. For compatibility reasons, as well as a desire to make the web secure, browsers will only support HTTP/2 over HTTPS (the spec states encryption is optional). If you want to benefit from this evolution of the web ? you need to switch to HTTPS.
Criminals are not the only ones looking to make money of your site ? Internet and WiFi providers want in on it too. As many as 38% of WiFi proxies, ranging from giants like Comcast to smaller providers, inject their own ads on unencrypted pages. If ads are how you make money, know those ads may be hijacked, and your users will be none the wiser. If your website is not using ads? Your users may see some anyway, and blame you for it. Use HTTPS to prevent such tampering and protect your revenue & brand.
HTTPS aims to protect your privacy, including not sharing with others what you?re browsing. Imagine you browse
https://secret.com/HelloKitty/ and click a link to the unencrypted
http://other.com/. If the request to other.com included the URL in a Referer (sic) header, anyone listening (as well as other.com) would know of your love for the little not-a-cat. To avoid such a violation, browsers do not send a Referer header when navigating from HTTPS to HTTP (unless explicitly overridden using a Referrer Policy). As more websites switch to HTTPS, staying on HTTP would hurt your insight into where your visitors are coming from.
Just to name a few.