Gears 2 is an perfect sequel
Single-player:I'm currently playing on hardcore difficulty and have not completed the game yet,but so far it have been a challenge,the AI is really smart and tries to charge you in all kind of ways.
Its not often you run into teammates but when you do,they often dies or gets lost.
Multiplayer: The multiplayer still got some lags,but its not as bad like it was in Gears1,more weapons,execution moves and meatshield makes everything more fun!
Horde:Is very hard even on normal,i have just reached to wave 14! And the bots are really smart.
War journal: Great way to keep the game longer by collecting important info and COG tags,also a way to unlock characters,you also unlock them by unlocked achievements from Gears1.