Much like the word “gravitas,” title case gives your words a feeling of formality and importance. Sites like The New York Times and USA.gov primarily use title case. It’s Professional. Serious. Established.
Using title case is like dressing your words up in a suit. For certain brands, you might want your words to look like they mean business. If you’re in the business of security, for example, title case is more likely to feel professional and trustworthy compared to sentence case.
Just as title case looks more formal and serious, sentence case looks more casual and friendly. I’m a writer at Dropbox, and we intentionally write in sentence case because we want our brand to feel natural and approachable. We think our product’s voice sets us apart from our competitors, and using sentence case is one way for us to maintain that voice.
I greatly prefer sentence case, for the reasons John outlines and more. I get irrationally bothered when people unnecessarily (in my eyes) capitalise words—particularly long strings of them—in an attempt to make things sound more ‘important’.
However, the title case example he presents does make some sense. His final thoughts are sensible advice for all writers and interface designers:
Title case and sentence case both have their advantages. Whichever direction you decide to go, just make sure you make an informed decision that makes sense for your brand. The worst thing you can do is to not have any standards at all, which eventually leads to inconsistencies that’ll be a pain to fix later.
Once your users start noticing inconsistencies, that’s when they start losing trust in your brand.