While initially combat will seem relatively simple, once you progress the game will get reasonably difficult, and you'll find yourself very carefully having to manage your cover, your shield/health level, and your surroundings.
Borderlands blends the regeneration aspect of many popular FPSs with health-pack grabbing. You have shields and life, and your shields regenerate, while your life doesn't. While this will give you some leeway to run into dangerous situations with guns blazing, you can't just stand still and blow everyone up. If you're going to make a rush, you'll have to take out your opponents quickly.
Some of the enemies are quite adept at sneaking around behind you, or quietly chucking grenades into your cover while you're trying to aim, and this can result in a quick death. Your FPS skills will definitely be put to the test in this game. You need a constant awareness of your surroundings, knowledge of all the cover available to you, some serious accuracy, and a quick trigger finger.
