Well, tomorrow I think about start such a topic but think that I need more testing. First impression:
- well, new special levels seems better. Randomness of paired levels is a great.
But lets look on 9.9.6 special levels. They are all challenging and make a chain of dependencies - you need complete one or two for (easy) completing another.
Hell Arena - challenge. You need to complete it if you want Rocket launcher (and break the Wall) and if you want get Arena Master Staff for breaking Vaults.
Chained Court - challenge. You face a strong AM who can resurrect enemies. You need complete CC for easy scavenge Vaults.
The Wall - challenge. Only Phobos-7 (and player level about 5-7), and for breaking wall you need previous win on Arena and get Rocket launcher. Reward not only backpack, but also Missile Launcher - great weapon for dealing with Bruiser Brothers on next level.
Halls of Carnage - challenge. Lava flood + former sergeants + demons + cacodemons/hell knights + closed doors. And BFG make things much easy on next special.
Hell Armory - challenge. No dependencies, but a good level for everyone. ...who can kill shambler.
City of Sculls - challenge. You need beat this for more easy handling corpses on Mortuary
Spiders Lair - of course challenge. At this point we have a master trait ready. MAc + nuclear plasmagun means no more ammo needed. Only some rockets... And nuclear weapon means nuke possibility, critical for Full win.
UC - every melee char must win that level for getting absolute melee weapon and...
Mortuary - give us a second part of set, armor. One of most challenging special level.
Vaults - challenge. Just good place for getting experience, some badges and rare weapons.
Lava Pit - of course challenge. And you need complete it and get reward for full win.
New special levels have not such dependencies anymore.
New Arena - good.
New Chained Court - yes! both layout, and without Arena Master it extremely good.
Phobos lab/Military base - both are nice. A good two levels, very "doomish" in style. But not so challenging (for that depth we are need some more dangerous) - and no missile launcher as a reward for next, Anomaly
The Wall/Containment Area - both are excellent but... for that depth we have something plasma/rocket firing and not so challenging anymore. And not need win on Arena for breaking them.
Deimos Lab - ouch! The best one. With insane difficulty and 2 shamblers. But... not equal Hells Armory! And not for that depth, IMO. Dangerous like opened Vaults.
Abyssal Plains - good. But... are you sure that Agony Elemental must be on ITYTD?
Halls of Carnage - at new depth it becomes easy, and paired with Spiders Lair???? Why? :( was nice have both (AND, not OR)
Limbo - excellent pair for Mortuary, no comments.
Mt Erebus - excellent pair for Lava Pits, no comments.