Reduction of the Old English diphthongs to monophthongs, and the emergence of new diphthongs due to vowel breaking in certain positions, change of Old English post-vocalic /j/, /w/ (sometimes resulting from the [ɣ] allophone of /ɡ/) to offglides, and borrowing from French.
Merging of Old English /æ/ and /ɑ/ into a single vowel /a/.
Raising of the long vowel /æː/ to /ɛː/, and (in the south) raising and rounding of /ɑː/ to /ɔː/.
Lengthening of vowels in open syllables (and in certain other positions). The resultant long vowels (and other pre-existing long vowels) subsequently underwent changes of quality in the Great Vowel Shift, which began during the later Middle English period.
Loss of gemination (double consonants came to be pronounced as single ones).
Loss of weak final vowels (schwa, written ⟨e⟩). By Chaucer's time this vowel was silent in normal speech, although it was normally pronounced in verse as the meter required (much as occurs in modern French). Also, non-final unstressed ⟨e⟩ was dropped when adjacent to only a single consonant on either side if there was another short ⟨e⟩ in an adjoining syllable. Thus, every began to be pronounced as "evry", and palmeres as "palmers".