I don't know why Nintendo even bothered. They took out the Online aspect of the game and translated it: there goes the most unique concept for Pokemon, down the drain. Pokemon Crystal is a remake of Pokemon Gold/Silver. It is also the 'latest' and 'newest' Pokemon game out now. This trend is getting pretty lame.
In terms of gameplay, the mechanics are generally the same. You play a trainer who aspires to be a Pokemon Master, and after a little errand for your local Professor Elm, you get a Pokedex from Professor Oak and begin your merry journey across Johto and eventually Kanto. The new tweaks are basically being able to play as a female trainer, an expansion to the Unown puzzles, a different way of getting Suicune, a new radio show, and new encounter rates and locations for Pokemon. There's also a new Bargain Corner at Goldenrod, some new items and skills, new items from phone friends and some story and map changes. An example would be the event in the Dragon Caves.
If you are wondering, no, you still can't catch all 251 Pokemon in Crystal alone. You'll still have to trade with older versions if you want to catch 'em all, so for all those of you who wanted to go solo, it's either a Gameshark or too bad. On the flip side, players can catch Celebi in Crystal.
The time Game Freaks makes a Pokemon game were you can catch all the Pokemon without having to trade for any would be a time when money grows on trees and cats bark. Trading is still done in the same old fashion via link cable. You'll also have to wait for the Time Capsule to be 'fixed' to trade with Pokemon games Yellow, Red and Blue. There's really no logic to why you have to wait to trade with Red, Blue and Yellow. People wanting a Charmander, Bulbasaur or Squirtle will have to wait until they are Ľ of the way into the game before they can trade for them. The game emphasizes trading and co-operation yet throws restrictions in trading with older versions. Waiting to trade is a dumb idea indeed.
The Pokemon will have some new techniques to learn in Crystal, but not enough to worth mentioning. Overall, Pokemon Crystal still plays much like Pokemon Gold/Silver, so those with the previous version will have no trouble with the game.
Graphics wise, the game has got a slight overhaul. That's probably the reason why you can't play the game on a normal Game Boy anymore; it's either a Game Boy Color or Game Boy Advance. The Pokemon have a brief animation before the battle goes into full swing, like Cyndaquil's fire bursting from its back to Pikachu swinging its arm in circles to warm up. It's all pretty enjoyable but gets repetitive and annoying over time, since it does make battles slightly longer. Technique animations are still exactly the same as Pokemon Gold/Silver, so you'll see all the same animations all over again.
The music as well, hasn't gone through much of a change: the same light-hearted tunes return. There are some nice new melodies, though, like the one you hear when you battle Suicune, Raikou or Entei. For those who have played Pokemon Gold/Silver though, you've already heard them all. The same goes for sound too; all those beeping noises make a triumphant comeback and Pokemon 'calls' are all still the same droning in Pokemon Gold/Silver. Not much of a workout for the Game Boy's sound chip, that's for sure.
The control is Pokemon standard. You move in the 4 basic directions at the pace of a geriatric slug. Getting the Bicycle in the game is a blessing, mark my words. The menus are still the same and convenient way they were in Pokemon Gold/Silver. Sorting items is a breeze as usual and really speeds things up a notch. Your Pokegear still works fine, allowing a clock, phone list, map and radio. Expect lots of battling due to the limited movement, though. Traversing a cave without repellant can be a serious pain.
Overall, if you have never played Pokemon Gold/Silver, then this game is a definite must buy. If you already own Pokemon Gold/Silver, it's really not worth it to get Pokemon Crystal unless you are a serious Pokefreak. The Japanese have an incentive in the way of online play, Americans don't, so the biggest draw for Pokemon Crystal has been removed. Blame the not-as-big cell phone market in the US if you want, it won't change the fact that the US Pokemon Crystal is just half the game it's supposed to be.