What gameplay effects change in multiplayer?

Solution 1:

I haven't noticed that change - phaselock can pull an enemy (including the ION loader) from behind the shield if you target an enemy just outside it, but it still can't target the enemy inside it.

With each additional player:

• Enemies become tougher.
• Rare (purple, fuschia and orange) loot drops become more likely.

The following items become communal - one gets it you all do:

• XP
• Money drops
• Ammo (you can now pick up ammo that you've maxed out on)
• Eridium
• Seraph crystals

Loot drops go to whoever picks them up first, as does quest related loot.

Also several items have effects dependent on the number of players equipping them, and many abilities are communal.