The differences are pretty significant. In general, there are a lot of detailed, fiddly mechanics in AD&D while BECMI is intentionally simple. The other most obvious difference is demi-humans were a class, so you never had a dwarven cleric or halfling fighter. This also allowed demi-humans to not have any level limits. Finally, BECMI capped at level 36, while 1E (not 2E) was completely unlimited.
Interestingly, the differences don't make that much of a difference during the game, so long as you were consistent with using either set of rules. This allowed characters to play in adventures designed for either edition. This was great for AD&D, since BECMI had some of the best adventures.