First and foremost, I define "spoiler-free" as not pertaining to the plot of Black 2 or White 2. If you don't want to know gameplay-oriented spoilers, such as which Pokémon are version exclusives, then this article is probably not for you. This article is here to make the decision for those of you who, like myself, tend to only purchase one of the two versions of any given Pokémon game, rather than both. I'll run down things like version exclusive Pokémon as well as version exclusive tidbits you might not know about. Disclaimers out of the way, let's get started!
Chapter 1: Notable Version Exclusive Pokémon, Or: Yes, You Still Need Friends to Complete Your Pokédex, Sorry Everybody.
Instead of running through each individual version exclusive Pokémon (which you can find on Bulbapedia's handy table,) I'm going to just point out some of the highlights. As usual, for each version exclusive Pokémon in one game, there is a related Pokémon that is version exclusive in the other. Black 2 has the Magby line, and White 2 has the Elekid line.
Well, sort of. That 1:1 ratio is true in most cases, as it is for Magby/Elekid, but for some reason, Black 2 gets three more exclusive Pokémon than White 2, namely the Gible line. I don't get it either.
As far as version exclusive legendaries, Black 2 gets Zekrom and Black Kyurem while White 2 gets Reshiram and White Kyurem. Regice and Registeel are sort of exclusive to White 2 and Black 2, respectively, but not really. It's confusing. Just know that you'll need a buddy with the other version to get your non-respective Regi.
Other notable version exclusives are the Gothita and Vullaby lines for Black 2 and the Solosis and Rufflet lines for White 2.
Chapter 2: Assist and Challenge Modes, Or: For the Wimps/Pros Out There
I'm really excited for this (these) feature(s.) Assist Mode, exclusive to White 2, makes your opponent's Pokémon lower level and controlled by less intelligent AI. In other words, the game is easier. This feature is unlocked by beating the champion in White 2. I could forsee this being used if you've got a younger player who wants to play your game but is easily frustrated by losing.
Challenge Mode, exclusive to Black 2, makes your opponent's Pokémon higher level, and increases the intelligence of the AI that controls them. This is meant to make the game tougher for the more competitive-oriented players out there. Like Assist mode, it's unlocked after you've defeated the champion in Black 2. I'm really looking forward to this. I'm not sure how much tougher the opponents get, but if it's Battle Subway-level, I'd be pumped.
Chapter 3: Miscellaneous, Or: I Know I Said Only 3 Things, but I Cheated.
Now for the smaller stuff. White Forest and Black City are back, in White 2 and Black 2 respectively. Simple enough.
As mentioned earlier, each version has a "sort of" version exclusive in Regice and Registeel. Once Regirock is caught, the player earns the Iceberg or Steel Key, depending on their version. Using said keys, the player can encounter Regice/Registeel. The key can then be transferred to the other version using a new feature called the "Unova Link," which also includes Assist and Challenge modes.
Finally, there's storyline continuity. Black 2 is continuous with the story details of Black, and White 2 is continuous with the story details of White. I'd say more, but that'd be spoiler territory.
So there you have it. Now you know, and knowing is...etc.