Well, the ones that can(even though hard to) be undone are mostly the Traits and Skills, I like the Perk System in fallout 4 and the Perks in Fallout 3 were shity, but in New Vegas you had more variety, the Ammo Variety was another goodie left aside, durability was a way better way to use the repeated weapons you found than just Hoarding to Scrap or Sell in Fallout 4 The ones that can't be fixed: Reputation System, with quite some side factions with their background history to discover and the variety of quests(helping the villagers or the Power Gangers, the Power Ganger or NCR, the NCR or the Legion and also different paths to complete the quests, which also brings the 'Being Evil' option or rather the Moral subjectivity(picking the lesser of two evils)) http://i.imgur.com/n3ZCaR0.jpg