I know that there are people who say "Gen I is the only good one!" but I'd really appreciate not stereotyping the entire lot. I'm a fan of Gen I. But my favorite is the Gen IV remake of Gen II. I also liked Gen III. Maybe you'd think that distinction is enough, but I'm still one of the people complaining about the modern Gens. Though I may just be one of the people who grew out of it. I'll probably eventually find out whenever X/Y releases.
I started playing Pokemon with Gen I. I still have my original Red cartridge, and for the time being it still works. I played Gen II, Gen III, the remakes of Gen I, and Gen IV was where I started feeling something was off. Now, I didn't mind Gen IV. I put over 60 hours into it. But you know what the problem is? I can't remember practically anything that happened during those 60+ hours of game time. I can still remember Kanto, Johto, and Hoenn. But the remakes of Gen II struck me right in the nostalgia and made me forget all of those reservations. Then Black/White came.
I couldn't even finish them.
And it's not because "the designs are uninspired" (though there were only a handful I actually liked). It's not because the gameplay is crap or anything (on the contrary, it's probably the best it's ever been; Though it could do with speeding up). It's not even because I was "bored of the same old story" (though the story factors into it).
No, I didn't like Black/White because in their attempts to recapture the magic of Gen I, GameFreak appeared to have forgotten what made the Pokemon games so compelling in the first place: The world. And everything packaged with it. Unova was such an, ironically, flat region. Its layout was nonsense, its Pokemon distribution was insane (seriously, no Water Pokemon in the wild aside from some shitty duck until five badges in?), and its pacing was abysmal due in no small part to the fact that GameFreak, or whomever does the writing of Pokemon, are about on the same level as Capcom and Sonic Team in writing skills.
If you really want me to touch on the Pokemon designs themselves, then I will. I didn't like the majority of them. Not because they were just ugly, but because they didn't make sense. If you look at Geodude or Onix, you immediately know what Type they're going to be. Dewgong, Articuno, Machamp, Gastly, Arbok, Golbat, Butterfree, Tauros, Rhyhorn, Dratini, Aerodactyl, hell, even Gyarados. Their designs and coloring immediately convey what types they correspond with. Even Gen II and III did it. Umbreon, Steelix, Ampharos, Poochyena, Ralts, Slugma, Camerupt, Skarmory, Electrike, Lotad, Makuhita, Sableye, Sneasel, Aron, Tyranitar, Meditite, Trapinch... sure, there were a few that make no sense, like Spoink, or Shiftry's half-Dark, or Altaria being half-Dragon, but they weren't the norm.
Purrloin is a Dark Type? I never would've guessed that. Roggenrola, Woobat, and Timburr my problems are two-fold with. Roggenrola and Timburr because they're uglier reskins of Geodude and Machop. Woobat because it's an uglier reskin of Zubat, and because it's part-Psychic for some reason. Why is Scraggy part-Dark? Archen and Archeops look ridiculous for being a two-stage version of Aerodactyl. Emolga is part-Flying? Oh, I get it, flying squirrel, great.
Admittedly it does have ones that do convey their typing, but they're spread out weirdly in the actual in-game world.
And then there's the evolution requirements... you know, I get wanting to have more to strive for past leveling Pokemon to 40 or so. I do. It's cool that more can still happen to them after they've hit the original 'peak' of 36 set by the Gen I starters. But the sheer number of Pokemon in Gen V that didn't evolve until after level 45? No. Just no. Especially for ones that only evolved once. Especially when you get them at relatively low levels in the wild. Dratini, Larvitar, Bagon, you found them at low levels and they took a while to grow into their final forms for a reason. I fail to see the reason for most of the Unova Pokemon.