What factors determine enemy levels? How do enemy levels scale to the player character's?

Question

I finished Playthrough 1 at around Level 36. I noticed that the enemies I find are now very easy to kill, meaning they didn't level up (or hit a level cap). I decided to started Playthrough 2 to fight higher level enemies and play the DLCs. I went to Jakobs Cove ('The Zombie Island of Dr. Ned' DLC) and found out that the enemies are at least level 42. I couldn't even get to the first objective without being eaten by zombies.

I noticed a similar thing in Playthrough 1 too: some missions and the enemies I encounter in those missions are of a higher level that what my current level is, making those missions a bit challenging until I level up.

So, for each of the playthrough modes, what factors determine enemy levels?

Specifically, I would like to know:

• I've played Borderlands 2 before Borderlands 1. How similar is Borderlands 2's enemy leveling system compared to Borderlands 1?
• Are enemy levels affected by what level a character accepted a mission? (Similar to Borderlands 2?)
• How does Playthrough 1, 2 and 2.5 affect enemy levels?
• Are there fixed enemy levels on specific areas or missions? What are the level requirements for an area or mission for each of the playthrough modes?
• Any enemy leveling differences specific to the DLCs areas or missions?

Answer 1

I don't have Borderlands 2, so I can't compare it. I'll just explain how it works for Borderlands 1.

First of all, in Playthrough 1 and 2, almost everything has a fixed level. Exceptions are DLC2 (Moxxis Underdome), which always scales to your level, and Crawmerax which always has a minimum level.

Things only scale after you've completed Playthrough 2; when you select Playthrough 2 you will get into what's generally called "2.5" -- no new quests or anything, but enemies will always scale.

Quests do not affect enemy levels at all. If (for example) you have accepted a quest to kill "super bad boss" in PLaythrough 2 at level 40, and you don't do it, then get to PT 2.5 and now decide to go there, the "super bad boss" will now be scaled to your level since you're in 2.5. The quest reward given to you, however, will still be at lvl 40.

DLC3 has it's own "PT 2.5" condition -- you have to destroy the Armory on the mission given from Athena (that's the first quest to destroy it, and also makes the end credits roll). Farming the armory is better done with the Marcus missions anyway. Until that, even if your main game is in PT 2.5, DLC3 will still be in PT2.

DLC2 always scales to you, no matter what.

DLC1 and DLC4 scaling follows the main game. However, the claptrap inventory increases are NOT give if you're in 2.5 (this can be worked around by using a multiplayer game in PT2 to hand them in, or just savefile editing -- Willow Scrubbery in particular is made for such fixes, as it figures out what rewards you SHOULD have automatically).