See here's the thing... I don't understand why so many of you who like the re-play value seem to be suggesting that it's either/or between that and the ability to play on open-ended after the main quest ends. Both are obtainable, and then we're all happy.
I'm not saying both aren't obtainable. I'm just addressing your comparisons to FO3 and Oblivion. These games didn't have replay value because there was little room to do things differently. Sure, having both would be great. The point I was trying to make is that FO3 seemed to have sacrificed replay value in favor of allowing the player to experience everything the game has to offer in one play-through. The quests are mostly on rails outside of things like Megaton and Tenpenny (huge decisions that have surprisingly little effect on anything), and FO3's character development system and lack of uses for many skills in-game left little incentive to try other character builds.
And you know what, in terms of storylines... was just thinking.... compare FO:NV to TES4. There were storylines in the original, un-expanded TES4 for the fighter's guild, the mage's guild, the thieves guild, the dark brotherhood AND the main quest. And there was every bit as much content to find by exploring the world, and plenty of small quest lines. In FO:NV we get ONE significant quest line/story, and lots of small ones that aren't all that interesting.
I won't deny that Oblivion had a lot of quests. The Dark Brotherhood quests were interesting. The guild quest-lines were pretty underwhelming, but that's ok. My biggest problem with the guild quest-lines in Oblivion (and most quest-line in Oblivion) is that they led to nowhere. I could rise to a leadership rank in guilds...and then what? A couple of nobodies acknowledged my rank in passing. I would have rather been given some kind of unique item or something if they were going to stop short of making these quest-lines lead anywhere...throw out the guild rank altogether and do something I care about. It was disappointing. Ideally there would have been some additional perks, responsibilities, quests, or something associated with becoming the head of a guild, but I would have settled for something more tangible. I'd much prefer to have fewer quests with more depth and player feedback than just having a longer laundry list of things to check off. The fact that I could become the leader of every guild in the game didn't help to make these quest-lines more weighty either.
FO3 had only a fairly short main quests and a handful of side-quests as well, and there were very few choices or variation involved in them. It did have more random "here's some thing for you to see" than NV does, but the game world, to me, felt less than thought-out and believable.
I'm finding NV to have a lot more questing, interesting conversation, interesting histories behind places and factions, interactions between factions, etc. than FO3 did. I can see how someone might think there's less meat to NV in the content area, but I don't. NV's world has a level of richness and justification that neither Oblivion's nor FO3's had, and that takes a lot of thought and planning. It's not easy. Not only that, but the quests in NV are so much deeper, multi-faceted, and full of choices that you really can't compare them to the quests in Oblivion and FO3. Heck, even Morrowind blew Oblivion out of the water when it comes to choice and depth in quests. It's a lot easier to create moar content when it's mostly unrelated to anything else and there isn't much interactivity or feedback for the player involved.
And, it's using the same engine as FO3. All that's fine, but the COST of the game is the same as FO3 and TES4 were. In many ways, FO:NV is a very large expansion to FO3, but we pay full price for it.

Here's where you completely lose me. Lots of games that aren't sequels share game engines. This does not in any way make them expansions. To be honest anyone that claims that NV isn't worth the asking price because it uses the same engine has no idea how much work goes into making something like that. What Obsidian did is not easy to do whether you get a legacy engine to build on or not.
If Obsidian made the game end because of the many NCR and Legion related quests around the wasteland that would suddenly be irrelevant, then they didn't think their story through very well. And to tell you the truth, there is nothing all that interesting about the FO:NV story... it lags well behind the main quest stories in FO3 and TES4.
So, simply building your story in such a way that it has no effect on the environment and NPCs is what good story planning is all about? Isolated factions, disconnected sub-plots, and little to no environmental after-effects are all good things? Interesting.
(Quick Insert: Gear as carrots is not irrelevant to games like FO3... Borderlands understood this.)
In that case Oblivion fails miserably. Gear was so irrelevant in that game that there was almost no point in dungeon diving for loot at all.
I'm going to stop comparing FO3 and NV to Oblivion, though, because Fallout is not post-apoc TES.
Finally, if you are not allowed to continue your character, then what is the point of DLC expansions?
I'd be absolutely amazed if you need to start a new character to play the DLC. Then again, they did it with Dragon Age, so...