Unlike many other things, Pokémon is something that improves over the years.
For example, in R/B/Y, all there was to do was going on in the adventure, battling trainers/Elite 4, catch Pokémon, Game Corner and Safari Zone.
When G/S came out, there were more Pokémon, more places to explore, the graphics were better.
When Crystal came out, it added the Battle Tower to the list of available things, and the sprites of the Pokémon were better overall, and they moved at the start of the battle.
When R/S came out, they had much better graphics (They dropped the in-battle moving sprites, though.) They included many Pokémon, but most weren't catchable. They added the contests, a feature that allows you to rebattle around 60 trainers in Hoenn, and they had a better Battle Tower.
When FR/LG came out, it dropped the contests (since they weren't in R/G and they wanted FR/LG to feel like R/G.) They had better graphics and more places to explore.
When Emerald came out, it had more Pokémon to catch, better graphics, and it had a whole Battle Fronter in it.
When D/P came out, they had much better graphics since it was on the DS. There was more Pokémon to catch, they brought back the contests, added an underground, more berries, more ways to catch/evolve Pokémon, etc. It was also possible to catch almost every other Pokémon in the game (Missing only the starters and legendaries of the 1st three gens along with Tangela and Tropius, wich were transferable from the previous games.) They also included the ability to connect over the internet to battle and trade with other players! Much more convenient!
When Platinum came out, it had better graphics, more trainers to battle, more Pokémon to catch, the Vs. Recorder features, glitches corrected from D/P, one more Pokétch app. to obtain, more Pokémon in the Sinnoh Dex, and it was possible to catch EVERY Pokémon existing except the legendaries (Not counting the birds and regis) from the other gens and the starters.
Now that HG/SS are coming out, they will include more touch controls, more minigames, overwhelming changes in graphics, new story compared to G/S, more events, Pokéwalker, the legendaries missing from D/P/Pt (Lati@s? Mewtwo?)