So what're the differences to Pokemon FireRed? Here's a little list:
-Play on the side of Team Rocket!
-Catch over 380 Pokemon, most of them catchable before the Pokemon League!
-Slight evolution changes for a few Pokemon: For example, Trapinch evolves at Level 30 instead of 35, but Vibrava will evolve at level 50.
-Relive Kanto as it should've been(at least for me)with new areas to explore and new challenges awaiting you!
-Gen 4 & 5 Pokemon!
-Pokemon which acquired special circumstances to evolve(like trading or happiness) will evolve more easily through level up(or evolution stones).
-Much humor and many references to real life happenings and characters! Some may like it, some won't, that's only up to you.
-Your adventure is far from over after the Pokemon League!
-The story has been noticably changed(isn't that obvious anyway?)!
-Difficulty has been increased as I think that the original FireRed(and by the way most of the Pokemon games are way too easy)lacks difficulty!
-Meet familar faces from other Pokemon games who'll be either friend or foe(rather foe, better than if it was the opposite, right?)!
Very important: Read this before you play!
-Make sure to get the National Pokedex or most of your Pokemon won't evolve! You get the National Pokedex pretty early in the beginning.
-Some road blocks will ony vanish if you got the right badge! For example the policeman at Route 107 will only disappear if you got 2 badges(Sometimes you need to leave and come back.
-Talk to everyone. Seriously, even if you don't like to hear(read) what Youngster Joey has to say about his Rattata, listen to the people. I hope you won't regret this.
-Open your eyes.Pokemon is about discovering on your own. When you've spent at least a month and you still can't progress then feel free to post your problem here.
After the main story: If you want to continue you need to have captured at least 60 Pokemon! and then talk to you-will-know-who. It doesn't matter if they're originally from Kanto or from other regions!
Update: Here are (probably) all changed evolutions, just in case you need them.
Golbat evolves at Level 40.
Evolve Poliwhirl by Water Stone/Sun Stone.
Kadabra evolves at Level 44.
Machoke evolves at Level 44.
Graveler evolves at Level 44.
Evolve Slowpole to Slowking by Water Stone.
Magneton evolves at Level 44.
Haunter evolves at Level 44.
Onix evolves at Level 36.
Rhydon evolves at Level 55.
Evolve Tangela by Leaf Stone.
Seadra evolves at Level 45.
Scyther evolves at Level 40.
Evolve Electabuzz by Thunder Stone.
Evolve Magmar by Fire Stone.
Evolve Eevee to Umbreon/Espeon by Moon Stone/ Sun Stone.
Shinx evolves at Level 15. Luxio evolves at Level 30.
Sandile evolves at Level 29. Krokorok evolves at Level 40.
Yanma evolves at Level 34.
Murkrow evolves at Level 36.
Misdravus evolves at Level 36.
Gligar evolves at Level 36.
Sneasel evolves at Level 36.
Piloswine evolves at Level 44.
Houndour evolves at Level 30.
Skorupi evolves at Level 40.
Silcoon evolves to either Beautifly/Dustox.
Ninjask evolves at Level 36.
Feebas evolves by Water Stone.
Carvanha evolves at Level 30.
Trapinch evolves at Level 30.
Snorunt evolves to Froslass/Glalie.
Dusclops evolves by Moon Stone.
Vigoroth evolves at Level 36.
Clamperl evolves by Sun Stone/Moon Stone.