Pikachu follows you around. Like in the cartoon, Pikachu doesn't like being in a Pokéball. So, it walks around just behind you. You can also check to see if it's happy, sad, or indifferent – just turn, face Pikachu, and push the A button. A window will pop up to show you how the electric rat feels.
Pikachu also talks. Yes, Pikachu does speak in Pokémon Yellow. Whenever you battle with Pikachu, it'll jump in with a "Pika!" When you ask how it's feeling, Pikachu blurts out a "pikachu" depending on his emotion. And, of course, the title sequence has a "PIKA!" as well.
Slight story change. Yup, to follow the cartoon story a bit more closely, there have been a few changes. First, you don't have your choice of Pokémon. Professor Oak captures Pikachu at the beginning of the game– and when Gary takes the only available Pokéball, Oak gives you Pikachu. There are other changes in the quest, but honestly I haven't gotten that far yet.
Game Boy Color support. The normal walk around graphics are still slightly bland on the Game Boy Color, but during the battles is where you can see the color enhancements. The creatures are now in full color, instead of the shade of blue or red depending on which version of Pokémon you own.
Game Boy Printer support. You can now print out facts from your Pokédex on the Game Boy Printer. It'll print out pretty much a pixel-for-pixel screen dump of the Pokédex entry – but you can only print out creatures that you've seen or caught.
The game still features the same 151 Pokémon in the past two versions of the game, but situations have changed that make it not possible to capture a few Pokémon in the game without the help of Pokémon Red and Blue.
Jesse and James, the two debonair baddies from Team Rocket will make a few appearances in the adventure, also to mirror the cartoon. And, to make sure that folks won't be traversing too familiar territory, items have switched around, some Pokémon will not be in the same place you remember in Pokémon Red and Blue…and others won't evolve at the same point, either. In fact, the Pikachu that follows you around will not evolve into Raichu.
