I started SoD a few months ago when they made all runes free, and I found it really refreshing how alt-friendly it was to level up multiple classes and easily get them raid-ready with gear from undermine reals.
I had multiple toons raid ZG/AQruins/MC/BWL within hours of reaching 60 and quickly get basically full T1/T2 epic gear. I recall my Warlock specifically was killing C’thun in hardmode with only like 6 hours /played at 60. This is good game design, accessible for multiple alts, and enables players both casual and grinders to participate in raids with very little friction. As a result, I have basically one of each class near an AQ40-gear level.
The Problem: This alt-friendly, accessible/inclusive gear progression runs straight into a brick wall the moment that you try to take a character into Naxx. 
Before highlighting the main culprit, it’s worth mentioning: Naxx already has a relatively steep, reputation-dependent attunement quest per character! Each character either needs lots of Argent Dawn rep (time-gate), or else lots of arcane crystals/nexus/righteous orbs (gold-gate).
If the above requirement were the only barrier to entry into raiding Naxx (as it is in most other versions of the game), then any character would naturally be able to go from Raiding AQ40 and having T2.5 gear into raiding Naxx and starting to get some T3 gear.
But instead we have this elephant in the room: the Sanctified gear and Seal of the Dawn progession grinds.
Imposed more strictly than gearscore for your BFD raids, most groups are effectively requiring that a character have their seal fully upgraded with a weekly time-gated currency, on top of needing to have a full set of “sanctified” gear, most of which comes from Naxx itself.
The Result: Characters that are basically in full 'Timeworn" gear from AQ40 are considered effectively useless in Naxx and won’t get invites unless they have grinded the NON-RAID content of Invasions and wandered around aimlessly in the darkness of Kara Crypts (also NON-RAID content).
So you go from steadily ramping up your character’s power level through raiding, to being told that all of the gear you acquired from previous raids might as well be a full set of grey Twill armor; because now you have to re-grind a completely new (non-raid) grind just to even get in to the next raid.
Again let me remind you that this is PER CHARACTER, and each alt must scramble up this broken ladder individually. Because the currency to upgrade the seal is Bind-on-account, it comes with the expectation that players must choose 1 main character to funnel all their currency/time to get the upgrades on. It’s no longer possible to just “jump in a Naxx raid” with your C’thun-geared character and start getting upgrades.
The Sanctified system is telling players “Pick a main, or don’t bother”.
For people that actually enjoy raiding multiple classes evenly and don’t have a set/designated singular “main” character, the Sanctified/Seal system is simply the death of alts having a reasonable progression. The Naxx attunement quest was enough, we didn’t need this gatekeepy “borrowed power” system that shuts out people who don’t want to waste their time wandering around Kara’s darkness mechanic.
Let alts raid if they are attuned.
I’m currently just skipping Naxx entirely due to this sanctified/seal b/s, and if we can’t join the new Scarlet Crusade raid with Tier-2.5 gear then personally I’ll just be quitting SoD. I’ve found it super fun over the past few months to level up and play with the class runes and hone in on which specs are working well, but if the intended gear progression for every character is that after raiding up through BWL/AQ40 we’re then supposed to grind invasions and Kara… I’ll just go back to Era again where any character can jump into any raid as long as their item-level is decent.
Not expecting anyone to miss me, and no you can’t have my stuff. But I write all this just to share where I’m at, and it’s explicitly due to the sanctified/seal path shutting out alts from raiding.