Gen 1 - The Original - Everyone was playing it and it was the new cool thing. Trading cards were sick and essentially embodied our childhoods Gen 2 - The Sequel - Sure it was cool that there were new pokemon, but for some reason it didn't hit as hard as it should have. People still liked it, but there was no craze over it. Gen 3 - The Upgrade - Gen 3 took full advantage of the Gameboy Advance and made one of the most amazing pokemon experiences ever. It vastly expanded upon the originals by adding new pokemon, weather related regions, mysterious legendaries. It was the ****. The craze was back. Gen 4 - The Fall - The kids who had played Gens 1-3 were now starting to grow out of pokemon, and because not many people were playing it, the people who were still playing it had no one to talk to about it. A lot of pokemon's power came from interacting with your friends; talking about your latest catch, that new legendary you just snagged, your epic E4 run, you know whatever.
Can't really say much for Gen 5 because I only just recently played it. I didn't play a pokemon game for 6 years after Diamond, then I heard about Gen 6 and decided to get back into it. So I really only played Gen 5 a couple months ago.
Gen 1- Buggy, but fun start. Pretty much everything, really. Gen 2 - So much disappointment. Gen 3 - Nice colors, good music, Pokemon becoming good again. Gen 4 - Thrilled at the physical/special split, yay skunk Pokemon, etc. Gen 5 - Fast battles, interesting story, so much "good", Skyla.
"Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod