I can't comment on why some people may dislike fallout 3 and/or fallout new vegas.
I personally played both to a great extent. I am still playing fallout new vegas. 491 hours on steam and counting for New Vegas, specifically.
I use a plethora of modifications. I feel that the customization of the game permits me to change things that I did not like about the game.
I also feel that the customization of the gameplay experience, from the system of leveling up and choosing your stats/perks all the way to the modifications I use,
permit me to make the game my own.
I couldn't imagine playing this game without modifications...
A few examples of the modifications I use:
--Sprint Mod: A mod which allows the player to (when the button of your choice is pressed) run significantly faster (based on formulae regarding carried weight, agility, endurance) for a short period of time until AP recharges.
--Implant GRX hotkey: Permits the player to bind the usage of Implant GRX (Implant GRX is a perk that lets the player use "Turbo" without having the chem) to any key of your choosing. Without this modification, I feel that the Implant GRX perk breaks gameplay flow by forcing the player to open the pipboy. Implant GRX was implemented with Old World Blues.
--MCV or Modified Combat Values: A modification which changes NPC behavior to make enemies more effective in combat against the player. NPC's crouch when possible to increase accuracy, they can search dead bodies for better weapons. (and frequently do) Also, NPC's actually aim thanks to this modification.
Default NPC's use a very strange "Spray and Pray" behavior pattern that basically means they never TRY to shoot you. This mod makes the game a lot more challenging -- especially if you piss off Caesars legion and 4 assholes with brush guns come at you in an open field.
The point is, the mods that I use in my game drastically change and improve the game world for my playstyle. Not many games let you do that~!
Finally, I'll just say this: I feel that the story, music, and "playable" areas of Fallout 3 were far better in my opinion.
The dynamic of Fallout 3 and its juxtaposition of the capital wastelands pleasant, if not irradiated countryside against the harsh, bombed out cities and sewer tunnels of the Urban sector made the game both visually diverse and made it seem much larger to me.
I found the lack of underground areas in Fallout New Vegas extremely disheartening, and while the shitty, barren desert most certainly succeeded in illustrating what a post-fallout life might be like, there weren't nearly enough buildings/places to go into and explore. It seems like 9 out of 10 buildings were boarded up and could not be entered. Fallout 3 had a shitload of buildings you could go into and explore.
Thankfully, "A World of Pain" puts over 100 rich, diverse, explorable locations into the game; including but not limited to an insanely large underground network of tunnels, pseudo-communities, lost-and-forgotten deep-underground facilities, and finally, plenty of above-ground buildings filled with enemies and loot.
I think that all sums up how I feel about the games in general.
I'm going to post links for the modifications that I have mentioned in this thread, as a courtesy to other PC players who would like to enhance their Fallout New Vegas experience. (I can do that, right?)
-- New Vegas Nexus: The site for Awesome, free FNV mods (www.newvegasnexus.com)
-- Sprint Mod: (http://newvegasnexus.com/downloads/file.php?id=34943)
-- Implant GRX Activator Hotkey: (http://newvegasnexus.com/downloads/file.php?id=42926)
-- MCV or Modified Combat Values: (http://newvegasnexus.com/downloads/file.php?id=40097)
-- A World of Pain: (http://newvegasnexus.com/downloads/file.php?id=38719)