If Champions of Zed is OD&D as it may have been if it wasn't rushed out the door without proper editing and consultation between the two diverse playtest groups being run by Gygax and Arneson, the Adventures Dark and Deep is a possible AD&D 2e if it retained the Gygaxian flavor that was lost in the actual 2e release.
ADD is, if nothing else, a clone of AD&D 1e with lots of new options and classes. It reads much like 1e to me just more user friendly, which is a compliment. ADD cleans up much of what was broken in Unearthed Arcana (sometimes known as AD&D 1.5) and fixes it.
Much of the draw of Adventures Dark and Deep are the new character classes, which can be easily dropped into any AD&D 1e / OSRIC campaign with little effort. I've always felt an affinity to Bards, and +Joseph Bloch has an excellent version included. The Jester subclass reminds me of the old NPC class from Dragon Magazine - I remember my sister playing one.
Cavaliers and Paladins are again class / subclass. Not the way I would run it, but the precedence is there from UA.
Mystic is a new subclass of Cleric. An unarmored cleric (but not of the fighting Monk type) this is the mystic of legend. "The mystic must live a life of self-denial and poverty". At best an NPC class with my group - as none will ever be able to keep this vow ;)
Savants are a new subclass of
Thief-Acrobats are back as a split from the regular thief class at 6th level. I didn't like the class split in UA and my opinion hasn't changed. I still feel it should start as it's own class at level 1, but Joseph is following Gary's probable intentions. I just happen to think Gary was wrong on this account.
Mountebanks are another subclass of thief trained as con men with some minor magic casting ability. I like it. I like it a lot.
Of course you have the normal list of classes from AD&D - less Assassins and Monks. No real loss losing the assassin class, but I do feel the absence of the classic monk. Sure, it wasn't really a European flavored class in the classic sense, but it helped define AD&D for me.
Joe introduces a skill system, which I'll get to with my next post in the series. I think the next review post will be the next part of the Five Ancient Kingdoms series of posts.