Hm, is it worth buying full price just for extinction? I'd say no. But do keep in mind that there are various other game modes asides from MP. You've got safeguard (which is like survival against increasingly difficult waves of terrorists), you've got squad games (basically you vs AI), and then you've got the campaign. The XP gained in Safeguard & Squad Games contributes to your MP XP - and I think that's a really great move by IW.
Now, if you can find a discounted version of the game - then yes, I'd say it's worth it for extinction. Many sales have been happening with Ghosts, so I don't think it would be too hard for you to find a copy at a fair price - used or brand new.
Onto your more specific questions:
- Yes, the game mode is good - it gives you the frantic feeling of zombies, but with more customization. You can choose from several roles (Weapons Expert, Tank, Engineer & Medic) which allows for strategic fun, as long as you have good team members. Communication is key, you need people to drop ammo, drop protective vests, drop equipment - you need to be in constant communication or else y'all will perish.
- The potential for a decent storyline is there, but keep in mind that at the time of Verrukt's release the storyline was still very fledgling. A lot of people say we shouldn't compare extinction to zombies, which I agree with to a degree - but I think it's fair to assess the timing of the maps release in relation to how much story we get. Nightfall clearly strives to flesh out the story A LOT more then PoC, and I think that's a very positive sign that the storyline will carry well across the DLC packs and probably into the next IW game.
- Nightfall has added "intel", in game quotes very similar to radios that you would find in zombies. Nightfall has also added a few new types of aliens. They also added customization to your character, to change appearance (which was not needed, but certainly welcomed).
- Longevity is still up in there air. With the first map, PoC, after you beat it - you were pretty much done. Sure, they have relics that make the game harder and give you more XP, but there is no REAL reward for using them. I still have not beat Nightfall yet, so I can't comment as to the longevity of it. Regardless, if you get into extinction, you WILL want to play it until you beat it.
- Waves/boss rounds go like this. You start off, and it is your job to destroy several lesser hives before you either encounter a massive hive or a boss. The more hives you destroy, the more difficult aliens start to emerge. At first it's just aliens that run at you and bite you, then it turns into nova 6 like aliens, then there are scorpions which can lob acid at you, then larger aliens, then giant aliens like rhinos, and finally in nightfall, they introduce a megalithic sized alien. Pretty intense.
- The game is not infinite, once you complete the last task, you'll be picked up by a helicopter - but not before having to run back to your starting point while a wide array of aliens are literally hot on your toes. It makes for quite a climatic experience.
- Apparently there is some sort of special ammo or weapon that you can use in Nightfall. Haven't gotten that far yet.
- The characters are meh. You don't know much about who you are, but with Nightfall - it's clear that 3rd party protagonists and antagonists are being introduced into the story.
Anyway, hope that gives you an idea and answers your questions. If you are interested, go for it - just don't pay $60 for it.