New:

HTML fragments like " class="something"/> and " alt="Img" /> are automatically ignored.
Strings with interpolated variables are now parsed ("Hello $thing").
Particular files and directories can be ignored. #1
Now supports installation via composer. #8
When there are errors detected, the exit code of the script will be 1 instead of 0.
Strings that are file path fragments are automatically ignored. #12
Strings that are MySQL query fragments are automatically ignored. #14
The ignores-tolerance can now be configured via the JSON config file. #15

Updated:

The HTML attribute name or the entire function call stack is shown in each error.
Child class methods are automatically ignored if the parent method is ignored.
Chained method calls ($this->method()->some_func()) are now followed and treated as a whole.
The WP_L10n_Validator::__construct() now accepts an array of text domains instead of just one.
Ignores defined in the JSON config file now override the default configuration. #6
When a l10n function is used without a text domain, an unexpected text domain ('') is reported, instead of a missing argument.
The cache files are written with pretty-printed JSON when possible. #11

Strings that begin with a leading underscore are now ignored with the rest of underscore strings that are ignored.
Method declarations are prefixed with the class name when checking against the ignores list.
Files which contain errors are always parsed, even if the cache shows that they haven't changed.
The parser is no longer messed up by complex variable expression syntax.
When the parser is unable to resolve a class name, it is reported as (unknown) instead of the method being treated as a procedural function.
The parent name of a class is properly detected when the class comes after a function declaration.
The ignores cache is updated instead of being reset when -c is used. #17
The cache is automatically invalidated when the configuration changes. #23