[The subtitles] are tiny, can’t read them at all. I don’t have 10/10 eyesight, but the text is indeed tiny, and it usually comes on white background that makes it even harder to read.
Ginji88, via Sleeping Dogs forums

The text is clearly readable on a black background as well as the initial speech tagged with the character’s name and color coded for the character. In the opening sequence, the ocean’s storm rocked Lara Croft’s ship, and the subtitles remained readable no matter the chaos occurring onscreen.
Megan Hammond, via Game Informer

The most common complaints about subtitle presentation are size, contrast, and the amount of text on screen at any one time. So ensure:

• Text is presented no smaller than 46px@1080p, either by default or though options
• Text is against a solid or semi-opaque background (known as letterboxing), ideally combined with an outline/shadow too
• Present no more than 40 characters per line, and two lines per subtitle (sometimes three, in exceptional circumstances)

Other important considerations include accuracy, positioning them at the bottom/middle and avoiding any other UI clashing with them, and using a clear easily readable mixed case (as opposed to full caps) font.

For subtitles to function, they must be readable, especially for the many people with hearing loss who have less proficient reading ability due to not having English as a first language. Art direction should not take precedence over readability.

However where the two requirements clash, both goals can be satisfied through providing customisation options – allowing players to choose whether they want a font that is easier to read, or one that fits best with the aesthetic.

Best practice example: Dead to Rights Retribution
Best practice example: Tomb Raider
Best practice example: Bertram Fiddle

More information: How to do subtitles well: basics and good practices