The main thing to say about Middle English is that everything got all messed up. Particularly once the Normans conquered England, thereby making English the language of the lower classes. A lot of the structure, spelling, and pronunciation that was left from Old English was lost during this period. There were no upper classes speaking English and "preserving" or teaching a "standard". Everything changed very quickly.

Characteristic features:

1.Simplification and reduction of inflexion.
2.Change in vocabulary due to introduction of French words.
3.The loss of grammatical gender.
4.Adoption of a more fixed word order similar to that of Modern English.
5.Expansion of vocabulary , use of French affixes.