I just finished Mass Effect 2 and I am ready to give my first impressions. I will just come out and say that this is probably the best RPG I have ever played. It's better than FF3, better than Chrono Trigger and better than Secret of Mana. There I said it. There are two main reasons this is so.
RPGs usually have two elements, fighting and plot/story development. Usually the plot/story development is the boring part. You need to read tons of boxes of static white text. Sometimes they try to spice it up and of course it helps if the story is compelling. Mass Effect's plot/story modes are the best part of the game. It is fun to have conversations and listen to the story. Mass Effect is fully voice acted, and when you are selecting which dialogue option you are going to choose it's only a 2-word summery. Basically you are choosing the attitude with which you will speak, and what Shepard actually says is a surprise to even you. So on your first play-through, you wont even have the opportunity to read what the other options are. This encourages a second play-through and still keeps it fresh.
The second reason, which is a genius BioWare invention (I think) is enemy scaling. Ever since I played Knights of the Old Republic I have loved this concept. It works like this, the enemies are scaled to your level to matter where you are. So you never have to grind to level up, and you never out-level the enemies to the point were the game is a cake walk. This allows the game to be really open-ended and allow you to complete the various story missions and side quests in any order you see fit and always have a healthy challenge. The is so good for reply value. Because enemies level up similar to you, higher level enemies have more skills and better weapons then low level enemies. So if you do quests in different order on your second play, you will encounter totally different situations. Parts of the game that you felt were easy will instead be hard because all of the sudden now the Krogans have incendiary ammo, or the Asari now have Shockwave.