What are some things that Skyrim took a step backwards on?
To me, and only comparing to past TES games, I'd say customization. They streamlined things to the point where there are basically three builds and the way perk system is you're forced to take perks that just add percentages (showing an utter lack of creativity) to be successful in that skill. The perks are less perks and just your actual skill as your straight skill level means next to nothing. This hamstrings creative builds. Also, whole trees like speech and lockpick are functionally useless. On top of that speech levels simply from selling things!?! Really? Who the hell thought that was a good idea.
Besides the mechanics the quests are linear, uncreative and, aside from a couple, pretty boring and short. The radiant quest system is a joke. If I wanted to play an MMO where I grind fetch quests all day I'd do that. Worse, your journal (which isn't even a journal as it has no notes) just fills with these spam quests just by talking to someone.
This leads to dialog, which again, is a joke. I can't ask anyone anything. Even important NPCs have a handful of lines that never change. Oblivion wasn't fantastic at this either but it was better and Morrowind was worlds better. The fact that you're forced to sit here and take insults without a way to respond is horrid.
Lastly, immortal NPCs. Literally hundreds of them according to the wiki. Inexcusable hand holding and immersion breaking. You cannot fail a quest through your own actions. And this is an RPG?
What ideas in Skyrim were good, but ultimately screwed up by bad writing/lazy design?
Companions are always fun just not the ones in this game. They're all bland and single dimensional.
The world is undeniably beautiful as are many dungeons. Unfortunately you keep getting sent back to the same dungeons and many of them have quest items in them that, if you pick up you early, risk breaking a quest. Also, far too few of them have any little self contained story that makes exploring feel worthwhile.
What makes New Vegas so replayable that Skyrim just doesn't have?
Simple. Consequence and freedom. I can do anything, kill anyone, betray anyone, and see what comes of it. The characters are so well drawn that I actually care what happens to many of them and they are actually memorable. Lastly, the quests. There are no go here, get that, come back quests that are ever actually that simple. There's always twists. You can almost always turn it back on the quest giver. The quest giver even can betray you. Then there are just beautifully creative quests that actually make you think and force you to make tough decisions.