Oh, my friend, they are like playing two different games. Granted, since you don't have Mass Effect, you are not aware of what you're missing and it's not a big deal. However, if you're like me and played Mass Effect and Mass Effect 2 on the 360, then played 2 on the PS3, there are some minor details that are missing. Here's a list (please note that I play solid Paragon):
Your inbox is filled with messages from characters you met in the first game explaining how they're doing. One man you helped get his wife's body back from the Alliance sends you a message saying he's opened a diner and soldiers eat free, thanks to you and his deceased wife. Another, a political person, contacts you saying he's doing his best to honor his word about helping biotics. He says it will cost him his career, but you can always count on him. Another, a Volus you meet when the Rachni are set free, says he's still alive and trying to recover, but things are hard. These are just messages.
--When you first meet Tali, you are given a third option where you mention you helped her get data on the Geth in Mass Effect. She immediately tells her squad to stand down when you bring that up.
--On Omega, you meet a Helen Blake, an ex-crime lord who you convinced to give up her ways in Mass Effect. She now works as a social worker, helping people on Omega.
--Also on Omega, apparently you can meet Fist, the info broker who sold Tali out in the first game. I never have, as Wrex kills him because I take Wrex when I go confront him.
--Once Garrus is recruited, you get a message from a lady he saved, asking if he's okay. She's talking about the time Garrus saved her in Mass Effect from a bunch of thugs that you walk in on.
--On the planet you go to get Grunt, you come across an Asari (whose name I forget). She is the same one you find on Virmire in the first game. It's actually a really funny scene. Her line as she leaves is, "Now, if you'll excuse me, I'm going to start running before you blow the entire planet to Hell." Kasumi's response is, "Well, she seemed nice. In a mad-science, ethics-for-sale kind of way."
--On Illum, you meet Gianni, an undercover cop you helped on Noveria in the first game. She finally buys you the drink she promised after you help her solve another case.
--Illum again, you cross paths with Conrad Verner, an obsessed Shep fave who is trying to act like you because he thought you were dead. He's overjoyed to see you again, and you can help him bust a criminal if you're nice to him.
--Illum, you meet Shiala, an Asari who was brainwashed by Saren in the first game. She's honoring her word and trying to help the colony stay on its feet. If you help her, she is grateful, and hints--heavily--that she's interested in you. Hoping she'll be a squad mate in 3. I think it'd be funny if Conrad and her turn out to be the best squadmates in the series, but you have to have been nice to them the entire time.
There are little dialogue options and references scattered throughout the game as well regarding you choices from the first game. Like I said, none of it affects the main story and if you don't know it's there you never would. But if you do know and it's missing, it takes a lot away from the story of the game, at least for me. It's nice seeing what characters from the first game are up to.
