In Invul, I'd say ResEl and ResEn are skippable, in that order of preference.
For DB, if you want to be able to do the higher level combos you really need to take all the attacks. I like having Taunt, too, but that's more personal preference.
My Characters
Knight Court--A CoH Story Complete 2/3/2012
In my opinion, the following invuln powers are 'non negotiable': Dull Pain, Invincibility, Temporary Invulnerability, Tough Hide, Unyielding. (Those are in alphabetical order. 
I'm not ranking them.)
Of the remaining four, I would rank them as follows, starting with the most useful: Resist Physical Damage, Resist Energy, Resist Elements, Unstoppable. I would not skip RPD without a really good reason.
Personally, I have all of them.
Dual Blades - I have no clue. Haven't done that set yet.
Paragon City Search And Rescue
The Mentor Project
Invuln: I'd take everything except Unstoppable.
DB: I guess best bet is to take Blinding Feint, everything needed for Attack Vitals and Taunt.
- @DSorrow - alts on Union and Freedom mostly -
Currently playing as Castigation on Freedom
My Katana/Inv Guide
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. -Einstein
^ Same as above, except I didn't get Taunt.
I got all on both sets except taunt but I´m thinking of respecing and getting taunt in exchange for unstoppable, I'd use it more often.
Working on a new tank and my first viable foray into invuln tanking (my main tank is a WP) and I'm trying to see what's skippable and what isn't for an invuln/db.
From what I'm reading, unstoppable is definitely skippable. But anything else? Looks to me like I'll want the rest of the set, yes?
And ditto for dual blades... I'll want the full set there, right?
Thanks!!
--Virtue--
My 50s:
Tigra Swipe (BS/SR/Dark Scrapper)
Galena Storm (Emp/Ele/Ele Defender)
Master Tolarin (Psi/Fire/Force Blaster)