a lot of the stuff in new vegas is from the originals, FO3 did a lot of stuff wrong and ignored a lot of the mechanics from the original games...
bethesada improved fallout , but you have to remember, they were introducing fallout to wider audience than the previous fallout games and the fact is they made a game that was just overall more fun and more popular than the previous fallout games, FO3 has a game rating of 9.0, and bethesdas other game elder scrolls oblivion is rated at 9.6, they're both similar in lots of ways, tons of exploring, plenty to do with countelsss hours of gameplay that keeps you busy for literally years, yeah obsidian did make some improvements like the reputation system, better voice acting, more dialogue, good complex story, but they ignored a lot of areas and the fact is about 50'% of the people who played new vegas and FO3 have noticed the lack of exploration, the lack of well armed human enemies to fight, the lack of of a dynamic world, its a very static game. so static it breaks the game, after you've played new vegas a few times, there is nothing new you'll ever see, no random encounters, no random events, the same creatures in the same areas, no factions ever patrolling the map, most of the map has nothing going on and its pointless to cruise around, cause after level 15 or so there isn't anything to do except finish the quests, exploration is non exisistent in this game unfortunately, don't expect FO4 to be like new vegas.