Berries are pretty basic and you can pretty much know what they do in their description. Some for healing, some are for giving boosts, etc. Generally I never use them as held items.
Abilities and nature are also like that. Natures add 10% more to a certain stat while reducing another by 10% too, although some natures are neutral. You can check what the nature does by looking up at the Pokemon's stats - red stat means it's buffed by the nature and blue stat means it's reduced by the nature. It's pretty ignorable in-game as it's mostly for competitive purposes. Abilities however come in different flavors and each Pokemon have different abilities with different functions.
You don't really need to worry about IV/EV in Pokemon unless you play competitively, although EVs are something worth noting, especially when they're much, much more flexible in BW2 and can help you in-game.
Effort Values (EVs) are "stat exps" you gain after gaining exp from a certain pokemon (e.g. rattata has 1 speed EV). For every 4 EVs you gain 1 stat point (so 4 speed EVs = +1 Speed). Each stat can only gain up to 255 EVs (although 252 is the more ideal one since remember, 4 EVs = 1 stat and you'd have 3 spare EVs left unused if you go 255), and up to 510 EVs in total. This allows you to pretty much give some neat statistical upgrades to Pokemon who technically are not good at them (for instance you can make Nidoking a powerful special attacker by maxxing out its SAtk EVs). Also, as I said, you gain them
after gaining exp from certain pokemon, so they don't have to deal the final blow - simply giving them exp share should be enough.
Vitamins (carbos, etc.) give 10 EV for the specific stat they mention but can only raise up to 100 EVs total. Wings give 1 EV for the specific stat they mention without the 100 EV limit. Some berries and even NPC services in Join Avenue reduces EVs to help you set them up properly. Some held items even boost EV gain.
Why I mention this for BW/BW2? Because back then, stat changes from gaining EVs were only visible after leveling up - that is why rare candies were looked down back then unless you maxxed out the EVs. You're out of luck when you reach level 100 and had not properly trained. Now in BW/BW2, stat changes from EVs already take effect the moment you get them rather than via level up, so you can pretty much see how very easy it is. There's no limit even when you get to lv. 100 as well, so now you can properly retrain Arceus (who is also always level 100 with EV spread evenly to all stats).
Don't be overwhelmed. Pokemon has always been a highly approachable series, and Black 2 and White 2 are no exception. The systems have evolved and become more complex over time but they've always been kept at arm's length (often to the chagrin of more dedicated players, even).
Huh? I disagree - a lot of competitive players like the new, eased up features.