The B/X rules sometimes have specific differences both in rules an information presented.
For example Holy water in Basic
Holy Water in Expert
you get two rounds of damage in Expert (talk about power creep).
Medallion of ESP in Basic
Medallion of ESP in Expert
I have no idea why the Medallion of ESP is different in the Expert rules as ESP is a second level spell described in the Basic book?
Wandering Monsters in Basic also have their alignments listed parenthetically after their name.
That are not present in Expert
I guess by 4th level you are just killing things without bothering to talk to them.