Please note this is based upon a few minutes of time in the Event Quest so I could be off base, but I am also looking at historical precedent.

Traditionally in DDO we set the adventure level on Normal, Hard or Elite. As Time wore on and the game got more popular we got solo, I mean casual and on some select few quests Epic.

Developers have talked about in the past how creating the scripting on each level was basically like a new adventure, short the art work.

When the game was first released there were dramatic differences in the difficultly settings. Not only were there HP differences, but different spell selections, different AC requirements, different DCs and really the higher you set a level of difficulty the more challenging it became (for characters at or below the level of the quest).

As the game has grown the level of difficulty in the different settings has decreased. There are reasons for this, including attempting to get more content out in a shorter amount of time, reducing bugs, and I forget what else the Devs have stated in the past.
This leaves basically the HP of monsters we fight as realistically the only thing that really changes. I mean a DC going up 4 points between normal and elite is nothing.

I am going to use Kobolds as an example, specifically Kobold Shaman from say “Waterworks” or “Surviving the Butcher’s Path”.
On Normal these pesky little creatures could hit you with Acid Splash (DC 10 d4), Alchemist Fire (DC 10 d4), Bane (DC10), Fear (DC 10) or Magic Missile (d4).
On Hard those spells get upgraded to Hold Person (DC 12), Obscuring Mist, Scotching Ray (4d3 +12) losing Acid Splash, Bane and Fear [at least not cast upon me during my test]. I should also note I took less damage from Alchemist Fire on Hard than I did on Normal (Hit you but did no damage.)
On Elite those spells get upgraded again to Curse and Lightning Bolt (DC 15 5d3+15) losing Magic Missile.

As we move up to Mired In Kobolds
On Normal Kobold Shamans hit you with Acid Ball, Acid Rain, Ball Lightning, Fear, Sound Burst and unholy blight. The DC appear to be 15, sorry I didn’t spend the hour or so to document this, as I did about two years ago with DCs and just verified them today.
On Hard the Kobold Shamans hit you with Acid Ball, Acid Rain, Disintegrate [Thrown Occasionally ~ approximately once every three/four times through the spell list], Sound Burst and unholy Blight. Again the DC appears to be around 15.
On Elite the Kobold Shamans hit you with Acid Ball, Acid Rain, Disintegrate [Thrown more often than hard ~ approximately once every two/three times through the spell list], Sound Burst and unholy Blight. I am not sure I failed any DC checks but hadn’t completed on hard for my lower level characters (below 15), so at some point I will go back in and check because +20 to +25 on the different DCs didn’t fail.
While the selection of spells change slightly the scaling of the damage against you doesn’t [there is something I bug reported, but] a cast of ball lightning that hits you for 15 points of damage still hits you for 15 points of damage as the difficulty increases. Or what is the difference between a Ball lightning at 15 points or a disintegrate at 15 points of damage?
The real difference in those quests is the amount of HP the mobs have. A generic kobold (in a full party) has around 150 hit points, 300 on Hard and 450 on Elite. Additionally the clusters may have another kobold or two on each level of difficulty increase.
IMHO the difficulty doesn’t really increase as dramatically in the newer content as it does in the older content.

When I tried out the new event the only difference I saw between level 15 and 20 was Hit Points of the mobs. It appeared the Mob DC were about the same and AC and …
Now I really didn’t have much time in the event to really note this but with the historic pattern of not making the level difficulty mean as much and the damage not scaling as much, just giving a mob additional HP concerns me as a boring and grindy way to make quests more difficult. I would prefer that Turbine went back closer to the original format of really changing up what the mobs did. I realize that could be a lot of work, but...

PS I hope I didn't post this twice as I timed out on the first attempt and don't see it.