So many bugs and crashes everywhere and all the problems you find in most Bethesda games. Combat isn't that fun and some quests are criminally underdevelopped.
And then you find out about the writing, you fall in love with this universe, its characters, all the difficult choices you have to do at every turn. Then you get lost on your 5th playthrough and find a city full of toxic waste that you never seen before and discover 3 fun quests inside it. The experience is guided so you find many fun things, but there's also great stuff under the surface that you may miss.
The evil army that destroys everything manage to be both scary and incredibly believable despite the fact they're cosplayers using quaterback equipement, the companion's stories are bittersweet and a pleasure to discover, then there's a Presley impersonator faction that is taking itself so seriously that I didn't realize there was a joke for an embarrassingly long time. New Vegas is a story about letting go of our past and accept the present for what it can give, and what we can give to it for a better future. It can be sad, funny, creepy, epic, inspiring, and sometimes all of them.
It really shouldn't be that good.