Your review of Forbidden City is spot-on. Nearly 30 years after I first read it I'm still amazed that no effort was made to actually write an adventure incorporating some of my favourite monsters of all time - aboleth, bullywugs and yuan-ti.
A great setting has been wasted by TSR releasing yet another incomplete adventure product but there is so much potential if the DM is prepared to write the adventure himself that you can almost forgive the publisher. Almost.
Can you imagine if the adventure hook about the jungle ghouls and the serpent queen had actually been the adventure contained in these pages? This adventure would have become a thing of legend.
Anyway, I'll get some value from it one day. I'm still determined to run it in 4E if I can think of a good way to fit it into the Forgotten Realms (I know where to put it and what baclstory to give it, I just can't think of the linking adventures I would need to get there).
Moldvay Basic was also the first D&D I DMed and the first D&D product that I bought (actually, I bought Gen Con IX Dungeons by Judges Guild at the same time). What a great game. And, yes, thieves were useless. Actually, less than useless. One of the worst things about pre-3.xE thieves is that they were essentially useless. While some would say that you just RPed them to extract value, you could simply take the armour off a fighter and do the same thing.
It's interesting to read Old Geezer's posts over at RPG.net where he describes Gary's original intention that a thief's skills only applied in extreme cases: for "normal" use, they would be automatically successful. Of course, that was never actually explained properly in the rules... and the utter uselessness was formalised until 3E was released.