A World of Pain is a lovely, large-scale mod that adds tons of dungeons, NPCs, and other content to New Vegas. Unfortunately, its writing needs a bit of help to line up better with the rest of the game. So, I did just that, and got into DLC integration and further fixes from there.
General bug fixes -- especially for AWoP users with no DLC -- can be found in the Interim Fixes file on the main AWoP page. I've migrated as many bug fixes to Interim Fixes as reasonably possible now that I am officially on the AWoP team.

FO3 Version

Weapon fixes: Most playable weapons in AWOP for Fallout 3 have received multiple bug fixes -- turning off nonfunctional iron sight animations, showing bonuses to Critical Hits/Damage/Chance/etc. in the Pip-Boy, adding pickup SFX, adding skill and/or Strength requirements, special VATS melee attacks, among other things.

NPC and performance fixes: All NPCs have had the "Can be all races" flag removed, as this flag's only use in Fallout is to have Dad's race match up with the player's. Including this flag erroneously on NPCs causes a noticeable dip in performance when there are many such NPCs.
Beast Master NPCs have had their voices and factions adjusted to act a bit more like normal raiders, and have some more proper combat barks.

Aid item fixes: Combed through all food items added by AWOP FO3, hopefully finally dealing with broken or inconsistent conditions and healing, whether or not the player has Food Sanitizer or Old World Gourmet. Many buffs were knocked down in efficacy and duration, as they were incredibly strong (and multiplied by Survival skill). However, AWOP food items now reflect their higher value compared to generic vanilla counterparts.

More: See the Logs section for details, note that some FO3-specific changes may be as far back as version 2.0.

NV Version

Large-scale revisions: All notes, dialogue, terminal entries, and quests from A World of Pain were reviewed extensively, and edited over the course of several hours. I tried to stay as true to the original content as I could for the most part, while adding and removing punctuation as necessary, fixing grammar, redundancies, all the ellipses("...") crammed in everywhere, making characters speak more naturally, et cetera. The vast majority of notes/terminal entries and dialogue have been changed in some way or another. I'm not perfect, but I think they're a lot better. Occasionally, I did rewrite larger chunks of content if I thought they weren't salvageable, such as the quest "where oh where has the legion spy gone?". NPCs connected to the For the Underground quest had their dialogue changed more than most, I'd say. All in all, there are hundreds, maybe over a thousand changes to these parts of AWoP.

NPC changes: All new generic NCR NPCs were renamed to be less of a mouthful. All of them, including Sergeant Arkus, had their 1st Recon Berets replaced with regular berets, for balancing and lore reasons. Some vendors, such as Hayward, have been adjusted to not offer you the world, with hopefully more to come.

Weapon DLC Integration: Applicable AWoP weapons may now be modified with GRA attachments, or repaired with non-unique GRA weapons (e.g. repair a Plasma Pistol Mk II with a Plasma Pistol(GRA) ). 

Rebreather Mk. II: A Rebreather formerly placed in an AWoP cell was replaced with a new Rebreather Mk II, made by myself. It is not a quest item like the original Rebreather, so it can be removed from your inventory if you so please. It has some extra bonuses, similar to all Mk II apparel. Its location is:


Challenge Integration: AWOP Weapons and NPCs have been made to count towards completion of vanilla challenges, e.g. to kill named Legionaries or commissioned NCR officers with certain weapons.

Exact Changes

The changes listed above are not all the changes I've made. You can find a more detailed list in the Logs tab. Due to how many changes there were to dialogue, notes, etc., I did not list every change down to the last comma this time.

Installation

I recommend Mod Organizer 2 (but just about any mod manager should work here). I don't recommend installing mods manually. Make sure that my patch is loaded after A World of Pain in your load order.

Consistent Pip-Boy Icons includes HD versions of most of the icons included in this file. If you already have that mod installed, don't allow this one to overwrite its textures.

A World of Pain fans may also be interested in the revisions I've made to A World of Pain for Dead Money.

Special thanks to Djmystro for making A World of Pain, and the many people who helped him get as far as he did -- see his credit section for an exhaustive list :)