#1761: moments created without a language are no longer following the global language, in case it changes. Only newly created moments take the global language by default. In case you're affected by this, wait, comment on #1797 and wait for a proper reimplementation
#1642: 45 days is no longer "a month" according to humanize, cutoffs for month, and year have changed. Hopefully your code does not depend on a particular answer from humanize (which it shouldn't anyway)
#1784: if you use the human readable English datetime format in a weird way (like storing them in a database) that would break when the format changes you're at risk.
#1785moment.momentProperties is now an array of properties instead of a hash. If you have a plugin that uses it to make sure cloned moments will retain the plugin-added properties you need to change your code.
deprecations (old behavior will be dropped in 3.0)
#1761lang is renamed to locale, langData -> localeData. Also there is now defineLocale that should be used when creating new locales
#1763add(unit, value) and subtract(unit, value) are now deprecated. Use add(value, unit) and subtract(value, unit) instead.
#1759 rename duration.toIsoString to duration.toISOString. The js standard library and moment's toISOString follow that convention.