Hi Rob,
I went through the first bit of your version of Dr. D's Den Hard. It was really cool, lots of intuitive ideas.
Here are my comments. Keep in mind you can do want you want and this is just my opinion, so don't worry about much. However, some of my suggestions where things should be this way are the way it should be in a proper custom mansion. (That I know of) There are differences between custom mansions where the designer can do whatever he/she wants, while in the built-in mansions there are guidelines to make it more formal and for the game, etc.
Anyway, I love the storyline with the goblin, really cool!
15695 - Red bird too fast for hard it seems
- Save point too early? (I noticed you have lots of save points. Not sure if it's necessary. Your mansion does seem a bit harder then a normal hard mansion, so perhaps so?)
15696 - Perhaps sync the platforms? They seem a bit sloppy, not offence. (Maybe two slow, two faster?)
15897 - A better hint for a bigger spider? It's really close to that lever there. Maybe have a layer six block gone so you can see its leg?
15896 - Quite hard to get the timing here right to get across. I would suggest dropping the skull spider.
16098 - Your ladders seem incomplete. There are no tops.

16097 - Your vines seem incomplete. There are no bottoms. :o I like this room. Good difficulty for hard.
16096 - Coming from the right the player doesn't have much reaction time stop before falling in the lava. I would suggest no lava pit + platform. Maybe just a spider.
16095 - Climb to the top of the vine, there's nothing there, but you can still climb farther then you should…
16094 - The platforms here are really cool, especially the way they go away, but I find it a bit strange for a built-in mansion. I would suggest deleting them and having the ground a continuous zapper beam.
16093 - Maybe it's me, but I must confess I am afraid of the dark (in MM). This room there is way too many ways for the player to die it seems and only one way to really win -- now I picked up the silver coins but died by dropping from the vine above it (the one on the extreme left) and could never know there was a platform there… This room seems a bit too hard for hard. I would suggest making it easier.

16293 - Freefalling in a built-in mansion I'm not sure about. I remember hearing from Vern somewhere it's really something from the custom mansion, and the tutorial says
Jack will NEVER be forced to fall more then his height
or something like that. I would suggest a bat platform to bring Jack down safely.
Background tiles in this section seem a bit messy, no offence. I would suggest cleaning them up a bit.
16894 - Too many goblins!
16893 - Too many goblins! Ahh!
16896 - This room is sweet. Having the floor switches suspended like that seems really really strange for a built-in mansion. I would suggest putting some sort of platform under it.
In this section it seems you're missing the heads of way too many vines.
16297 - If you climb up the vine without a head you will run into lava in the next room. (Ouch!) I would suggest putting a top onto that vine.
- The platforms should be above the lava. You can't see them that well.
16494 - Spiders too fast.
16498 - Are you meant to come back later for that red key? Oh I see, you come through that door… cool!!
Nice so far but seems to me it needs a lot of polishing and cleaning up graphic wise, in some of the puzzles and dark rooms. When going through this I get the faintest impression that this is a custom mansion… sorry! It's probably just me. I'm picky, I guess.

Basically I think a built in mansion should seem quite formal, right. This mansion is nearly there, but needs a bit of work with making Jack feel safer all the time.
That's all I can get to tonight, I'll come back to you with some more feedback tomorrow, perhaps.