I like this one.

I'm the first to comment and don't want to post anything controversial even though I have no controversial views it's just a good spell.

Cool !!!!!

REPORTED!

j/k, congrats, fam.

Since any creatures within the area of effect take damage — even those that would not normally take damage from sunlight, such as humans — one can assume that this magical sunlight is significantly more intense than ‘regular’ sunlight (12d6 more intense!). That being the case, I’m surprised that the spell doesn't specify increased damage for creatures with sunlight vulnerability (eg. vampires, of course). True, undead get disadvantage on the save, but that's ALL undead, not just sunlight-sensitive ones.  

Same could be argued for the Dawn spell in ‘Xanathar's Guide to Everything’.

Quote from Scrim >>

Since any creatures within the area of effect take damage — even those that would not normally take damage from sunlight, such as humans — one can assume that this magical sunlight is significantly more intense than ‘regular’ sunlight (12d6 more intense!). That being the case, I’m surprised that the spell doesn't specify increased damage for creatures with sunlight vulnerability (eg. vampires, of course). True, undead get disadvantage on the save, but that's ALL undead, not just sunlight-sensitive ones.  

Same could be argued for the Dawn spell in ‘Xanathar's Guide to Everything’.

 Vampires already have baked in sunlight sensitivity, and this spell gives them disadvantage on the save. Adding another effect that singles out vampires would be triple dipping and, frankly, overkill.

Am I the only one who's upset that Clerics don't get this, along with Sunbeam? It just doesn't seem to make any sense.

With Tasha's they do. You need to turn on the optional class features and additional spells.

EVERYONE! LEND ME YOUR ENERGY! (Sunburst is a spirit bomb)

should this really be a 8th level spell because a 7th level spell  already does this (upcasted fireball or delayed blast fireball)

1. This spell does radiant damage, which a lot fewer creatures are resistant to. 2. This spell can blind creatures. 3. The light generated by the spell is sunlight.

I think they should have added creatures with sunlight sensitivity to the list of creatures that should get disadvantage to the saving throw. 

This spell is an amazing case for giving flavor to a spell. I love to describe creating a small sun which I toss into a fray of enemies. If you know Escanor of Seven Deadly Sins, you know what I have in mind. 

Does this last until dispelled?

"Noooo, you can't just kill all of us at once with a massive burst of Radiant damage!"

Who Decided That?

I am not sure which part you refer to. So I try my best to go through the effects:

• The damage happens instantaneous -> nothing to dispel
• A blinded creature is blinded until it succeeds on a saving throw. I think there is room for interpretation if that effect can be dispelled. I would argue that it is not possible to dispel the blindness because there is no spell constantly blinding someone. The situation is more that a flashing bright light blinded someone with extreme force once which results in temporary blindness. Lesser restoration or any other appropriate healing spell would solve the ailment.
• Any darkness is dispelled in the area Sunburst is effecting. This effect happens instantaneously and does not effect any darkness that is cast afterwards.

My goblin antagonist killing the entire party with one spell:

I SAY OOOOOOOOOOOOOOOOOOOOOOOOOOOOH!!! Im blinded  by the lights

The Sun is a deadly laser~

Yeah might have been a mistake to let my Homebrew monster cast this at will with maximum damage and a +23 to hit. 

This is also WAY bigger than fireball