One thing specific to Black and White: How easy it is to be completely over-levelled between Nacrene City and Opelucid City. For example now i have two gym bages, currently chasing Team Plasma through the forest. In my party I have lv.18, lv.19, two lv.20 pokémon and a lv.16 pokémon deposited in Daycare. I find it really easy to keep up them high levels and whilst it makes trainer battles kinda boring, some gym leaders are still hard (electricity anyone?), and provided I stick to 6 pokémon for Drayden's Gym and beyond I don't become too much stronger/weaker than Ghetsis. With 151 new pokémon, using about 10 as part of my team at different points, not just as HM slaves, helped get used to it even faster :)
Also, although I haven't played B2W2 or beyond so maybe they're not as good...Team Plasma. They're the most interesting team there is (or more accurately N is just the most complex character in pokémon), and it really makes Magma/Aqua/Galactic sound like a bunch of maniacs and Rocket look just unimaginative in their goals.
That you battle N in place of the usual Champion battle after the Elite Four was really cool - I liked how the climax of the story didn't happen after badge No. 7 like usual, having to trek all the way through victory road and the elite four to reach the boss seemed a lot more worthwhile than destroying a load of grunts in a secret base. Plus, it's the only game that actually offers an explanation for the Champion reclaiming their spot - they don't spell it out, but whilst N beat Alder and you beat N, N didn't actually have the gym badges I don't think, so thus didn't count. Therefore you don't become Champion like when you defeat Blue, instead Alder regains his position, trains heavily after being walked over by a Dragon your character and N can easily capture and command, and thus presents a formidable challenge post-game.