An amazing experience
Lets put the good things out in the open, Mass Effect 2 is bigger, better, more polished, it has way above average graphics even on not so good gaming rigs and the overall tone is more engaging and more immersing.
The game play differences between the two are pretty obvious when you finish one and start the other, but after you get used to the more snappy and responsive controls you will find yourself not wanting to go back to the more paced original, the movement is smooth, the shooting is quite rewarding specially when you can chain different abilities and powers of your teammates.
The UI is clean and minimal, which is one its shortcomings, theres not much place for character customization except for your armor, weapons are streamlined and you cant really change them much like in the first game, youd expect alot more fiddling with technology in a scifi game, this was disappointing but didnt break the game.
The mako is gone (yes!) and exploring the galaxy is alot easier and less time consuming even though its rather boring mining for resources through this probe system.
But what makes ME2 a great game is the story, and the ability to make you want to be Shepard, you will lose yourself between the many possibilities, your potential relationships, and even recruiting is fun, some of the characters you will meet in the game are just brilliantly done and still you feel the urgency of the main quest wherever and whatever you do, but it seems you just had enough time to do what you wanted before going into the fray without feeling like the galaxy really didn't need you, they did and they will again very soon.
ME2 and ME in general have a problem with the alignment system, it doesn't seem to matter that much which is a shame.
But overall the game just feels awesome and hits the right spot, even if you try hard enough you wont find anything quite like Mass Effect 2.