The skulls are:

• Juggler - no ammo will drop for your equipped weapon
  
• Anger Issues - enemies are more aggressive
  
• Angry - enemies will not flinch
  
• Arc Burn - increased Arc damage
  
• Solar Burn - increased Solar damage
  
• Void Burn - increased Void damage
  
• Lightswitch - enemies have increased melee damage
  
• Nightfall - If all players die, you go back to orbit
  
• Heroic - more, more powerful enemies that are more aggressive
  
• Mythic - same as heroic, but an additional enemy from each faction gains a shield
  • Fallen - Shanks gain a Solar shield. (nb. Captains have Arc shield)
    
  • Hive - Knights gain an Arc shield. (nb. Wizards have Solar shield)
    
  • Vex - Harpies gain an Arc Shield (nb. Minotaurs have Void shield)
    
  • Cabal - Psions gain Void Shield (nb. Centurions have Solar shield)

You can't die and have less easily acquirable ammo, to enemies that are more numerous, more powerful, more aggressive and more shielded than before.

But your elemental weapons do more damage too. So you got that going for you, which is nice.

Good information, thanks. It might help to indicate the source of the info and how to actually get these into gameplay. Is there a way to manually enable the modifiers, or is it auto-selected for us?

Context is important. :)

And apologies if my questions seem obvious -- still level 19 and haven't finished the story mode yet, so maybe this all becomes clear once I hit 20…

Same. I'm wondering if this is something I can turn on, or if it's just applied to certain playlists/missions on Bingie's end (for instance, I think there's a heroic strike going right now which has Arc Burn enabled).

Good information, thanks. It might help to indicate the source of the info and how to actually get these into gameplay. Is there a way to manually enable the modifiers, or is it auto-selected for us?

Destiny Limited Edition Strategy Guide. Page 80. BradyGames 2014.

Though I rewrote the descriptions to attempt to skirt around any... issues.

For now, the Heroic skull is enabled on story missions when played above their default level.

The daily story mission, viewable on the left side of the director at level 20, also has extra skulls enabled. Today's has 'angry', I think. Yesterday's had 'Lightswitch.'

The weekly strike also has extra skulls enabled. This week's has 'Arc Burn.'

I'm assuming Bungie will mix it up in the future and, given their legacy, will eventually dare players to try it with all of the skulls on. Whether or not we'll get to play with them ourselves, I don't know.

Look forward to playin--er, watching you play with these on.

The weekly strike also has extra skulls enabled. This week's has 'Arc Burn.'

I'm assuming Bungie will mix it up in the future and, given their legacy, will eventually dare players to try it with all of the skulls on.

All 'skulls' on would almost be a buff if every burn element is turned on. After wrapping my head around the fallen doing insane damage in the weekly strike, it wasn't that hard if you kept your distance. Arc HMG, fusion rifles, and snipers just wrecked, and also with all of them on everybody's super is guaranteed to be way better.

Everything being hyper-lethal is better than bullet sponges. Assuming you're using the right damage type...