My experience deals mostly with scripting issues and lots of them. Some are BUGS (choosing the vegetation formula during "I could make you care" if you got to Vault 22 already [on behalf of the NCR OSI] stops the quest dead.) some are DECISIONS of design that are best described as . . . imprudent (the infamous BoS computer virus isolation free quest, and the endlessly non-entertaining NCR monorail.)
Lots and lots of gremlins too: finished quests still listed (all grayed out), arbitrary notations of dialogue topic for free quests, quest stages vanishing before my eyes (Corp whatsisface told me he'd like my help to arrest Keith, stood rooted in place, started walking very slowly with his gun held beside his hands (wish I had a screenie, it was pretty funny!), got to the front of the building then turned around and told me: "Can I help you?" = quest abortion).
Every hour of play bring up these things and it's fiendishly easy to break the flow of quests and info. None of it is utterly game-breaking but that is only because there is so much to do: when something breaks (as with Keith or "I could make you care" above) I just move on to the next quest and wave the XPs goodbye.
Again, not game-breaking but plenty immersion-breaking. NEW VEGAS needs one of those small scripting patches that contain a massive fix list.
P.S. I did some quest scripting for OBLIVION and it was complex but not complicated. I didn't think it would be so easy to mis-flag so many quests - I wonder if they tried to mess with the basic check/if/next structure of the engine. Also: I don't know how they modified the engine.