i preferred first cavalier as presented in dragon - most of us used spells and items and some good race/class and multiclass choices - none of class stuff works in my DnD BX varient
I would never use any of the character classes, weapon specialization (unless I were to heavily modify it - for starters, I would remove any damage bonuses and leave only 'to hit' modifiers; the changes would have to be so extensive, though, that I don't think it is worth even starting on the project), the new armor types (bronze plate? really? plus, I can't stand field/full plate), or comeliness. Most of the spells would be OK, but there's a spell that can cause a save vs. death and only uses up a first-level spell slot, and there are a few other problematic spells as well. Some of the magic items are ones I can't stand, like magic elven chain. I'd probably just tank the entire new magic armor table in favor of using only the original table. The new weapons (both in the normal and magical types) and the other new magic items seem workable to me, though.
Oh, and I've said it many times, but it bears repeating: the replacement weaponless combat systems, especially System I, are infinitely preferable to the horrible method in the DMG.
I rather liked the hierophant druid and the theif acrobat. I did enjoy the spells.I am not likely to use the barbarian or cavalier.
I have to say I like the cavalier. I'm running one in a 1E game my friend is running, been playing him for awhile. But, I think I am a special case and haven't gone gonzo with some of the abilities. I like the barbarian--sorta--as well as the thief acrobat. I would SEVERELY restrict the spells and magic items and even those who could be a cavalier or barbarian. The main issue with the barbarian is the destroying magic items and gaining xp. I have a few ideas I would use, if anyone wanted to be one.
I was going to say nothing. But actually, I allow:(1) The demi-human deities.(2) The polearm descriptions.(3) Some of the magic items, especially the +1 dagger.Certainly nothing from the first half of the book, though!