I am working a word based game. My word database contains around 10,000 english words (sorted alphabetically). I am planning to have 5 difficulty levels in the game. Level 1
There are several factors that relate to word difficulty, including age at acquisition, imageability, concreteness, abstractness, syllables, frequency (spoken and written). There are also psycholinguistic databases that will search for word by at least some of these factors. (just do a search for "psycholinguistic database".