Bigger, Louder, More adventurous, not always necessarily better.
The cover mechanics are largely fixed. The enemies more varied, but dare I say easier to kill than in the first game (Boomers are much easier to down, for example). I played it through on normal, then hardcore and then insane. Insane is most definitely very, very, very difficult to beat, but gives you a great sense of achievement when completed - the game has to be played on this setting to fully appreciate it.
You are taken to many different and varied locations than in the first game, and the visuals are bright and stunning, compared to the sepia feel of Gears 1. Many of these locations are on a huge scale, locust cities, underground caverns and outdoor moutain scenery, compared to the destroyed inner-city claustrophobia of the first game.
The vehicular sequences are the most difficult to get used to, and one feels that the creators are trying to please everyone by including so many of them this time around. The tank sequence is weak, as the only vehicular sequence of the first game was very weak. However, riding reavers and other beasts is really fun, and once you get used to it, really very exciting - the reaver chase and Hydra fight towards the end of the game is heart-pounding action reminiscent of the Speeder-Bike sequence in Return of the Jedi.
The finale is truly awesome, and without even getting started on the amazing multiplayer games - Horde and warzone being my personal favourites, you can revisit this game many times and it doesn't get boring.
This game is a must-play for anyone who appreciates beautiful visuals, realistic audio, great voice acting, fluid and satisfying gameplay (gruesome executions!), and a real adventure.
Buy it and Play it!