Last night, I finally bought SW:TOR and was excited to play it when it finally finished downloading. When I was watching the opening cinematics, I saw that there were no subtitles. I am tired of seeing MMOs exclude people like me, who NEED subtitles in media in order to enjoy them.
Miss PewPew, via MissPewPew.com

Accessibility features are super handy for everyone. I use em all the time – eg I occasionally play console games on mute in the morning
Christer Kaitila, via Twitter

Subtitling is one of the most commonly seen accessibility features, with some publishers (such as Ubisoft) even requiring it. It benefits a huge range of players, not only those with permanent hearing impairments, but other conditions such as ADHD, also if the spoken language is not your own first language, and situational impairments such as playing in a noisy environment, through poor quality hardware, or having to play on mute to avoid waking the baby.

In addition to ensuring that all important information is covered, also include flavour speech from background characters, speech in cutscenes, menus etc, to ensure that gamers who for any reason do not have access to audio are not only able to play, but able to have as close an experience as possible to other players.

Additional speech beyond that which is important to narrative or being able to play can either be included by default, or through a separate toggle.

Best practice example: Destiny 2
Best practice example: Dragon Age: Inquisition