If you've been keeping abreast of Mass Effect's development, you'll know that Bioware has been making quite a fuss over the conversation system, and with good reason. In some respects it looks like every other conversation system you've seen, where you're given a selection of responses to choose from when talking to an NPC, but as soon as you start playing you'll realise that it is very different indeed. For one, Shepherd never responds with the words you've selected - basically each selection is a framework for the type of response, rather than a verbatim sentence. This makes for some interesting encounters, and far more fluid communication.
Another great thing about the conversations goes back to those awesome facial animations. Characters don't have to tell you that they're angry, scared or sceptical, you can see it on their faces without them saying a word. If someone looks uncomfortable when you're questioning them there's a good chance they're hiding something, and you may want to push them a bit harder for answers.
Also, how you choose to communicate with people affects Shepherd's reputation, and will have a bearing on future encounters. If you regularly try to intimidate people, your reputation as a hard ass may scare the next person you meet, making them more pliable. On the other hand, if you try to strong arm the wrong guy, he might clam up when he could have been helpful.
As great as the conversation system is, there are still some annoying issues that I was surprised to see. For example, you can ask someone a question and get a prescribed answer, then if you talk to them again, you can ask them that same question and get the same answer. I would have thought that Bioware would have made sure that once a question has been asked and answered, it would no longer appear as an option.
A more serious annoyance, is that you sometimes can't ask the question that you'd really like to ask. Early in the game someone tells you something very important about Shepherd's Captain, but when I tracked him down, I wasn't given the opportunity to question him regarding what I had found out. Instead I had to wait until later in the game when he randomly decided to discuss the subject with me.
As with most role playing games, you don't wander around by yourself. Instead you have two other characters in your party - don't get too attached to one of the guys in your first party though! At various points in the game you get to change your party, and you'll soon realise how important it is to get the balance of skills right. There are basically three main classes - Solider (weapons specialist), Engineer (good with machines and computers) and Adept (equipped with handy psychic powers). There are also three subclasses - Infiltrator (Soldier/Engineer), Sentinel (Adept/Engineer) and Vanguard (Soldier/Adept).
If you choose your party unwisely, you can find yourself in deep trouble, something that I have first hand experience of. I set off on one mission with no engineering skill in my party, and therefore couldn't unlock any lockers, or use any medical stations - the upshot being that I found myself dying all too regularly because I had run out of medical kits and wasn't able to find anymore! But this isn't a fault of Mass Effect, it merely highlights my naïve tactics. I didn't make that mistake again though.