It's best to start out by stating two facts.
First off - Bioware is one of those rare game companies that are almost entirely reliable. In fact, only Blizzard are comparable in terms of consistency. Which means that their games are almost always - Neverwinter Nights offered a fairly poor single player experience - fantastic... it's something gamers appreciate and something that is incredibly rare.
Secondly - Mass Effect IS a good game. Criticisms which are about to be laid at its feet are not impediments to that fact... merely things which stopped it being quite as epic as it should have been.
The first thing to point out is - Mass Effect doesn't optimises your configuration... initially this made it seem as if it was an uber whore, putting Sanginus Nox - a machine that managed to cope admirably with Crysis - into jerk city... Fortunately this turned out to be because the audio was running on software alone... once that was sorted the game ran just fine. Also, your experience with the DRM may vary... but as usual, it seems to have been more of an impediment to legitimate players than pirates... Clearly it's asking too much for publishers to learn lessons.
The premise of Mass Effect is fairly standard - you're trying to stop the run of the mill bad guy from destroying the galaxy. It's more elaborate - naturally - but it's hard to go on without spoiling it. Although, the story doesn't develop so much as it is spilled out by various bouts of exposition. This is probably one of the main flaws of the game... you never really get the feeling the plot is developing, you just blunder from one plot update to another. Not only that but if you've played Knights of The Old Republic, there are some definite similarities. Although, that said - while the premise it uses is somewhat cliche, the universe it's set in is definitely interesting and is probably one of the strongest aspects of the game.
It's inaccurate to dub this game an RPG... at least in the traditional sense... the game is more Deus Ex than KOTOR in that there's no RPG style tick-tock round combat - which KOTOR contained... albeit in a rather unobtrusive manner. Although, it's third person and of course, you can pause to issue orders to your squad. At points the game actually manages to give you what are fairly realistic firefights. Which is to say, unlike in an RPG when range attacks will be exchanged briefly before everyone runs at one another to get within melee range - if you break cover to try and run up and bash people on the head here or just stand out in the open, you're probably going to end up dead and unlike KOTOR, if your main character dies in a fight - game over. This can be a little irksome but at least makes sense as the control you have over your squad mates is fairly loose. Which can become frustrating when they stand in the middle of a room and die, overall it's fairly passable - although the enemy AI does occasionally have a propensity to stop hiding and just run at you or try and circle strafe at point blank range.
The combat itself has configurable difficulty, ranging from the hand holding "point in the general direction of something and it falls over dead" to "hardcore gamers only", presumably the options are there so that those not au fait with more realistic, less tick tock combat are not frightened by a game that does occasionally require reasonable accuracy and reflex - at least on the higher settings. Of course, the options included allow you to determine whether the combat is a formality or a big part of the game. Regardless, it's a fairly intuitive system and barring the occasional idiocy of your squad mates it's quite enjoyable. As you'd expect, weapons and armour can be upgraded and there are variety of items out there - some better than others. Rather vexingly, weapon skills are class specific - so, if you're a Vanguard, no matter how good you get, you'll never be able to skill up with an assault rifle. You can still use one but even crouching down on a single shot basis with no wind, you'll be lucky to hit the broad side of a moon. Also, there's a notable absence of melee weapons... you can run up and pistol whip people but no discernible melee weapons but then given the combat dynamics of the game, they'd probably be the equivalent of suicide for all but the most heavily armoured and shielded individuals.
An interesting aspect of the weapons are that you can buy licenses. Basically this means that there are some merchants who will sell you a license and that the vendor on the Normandy can then stock that brand of weapons or such - as there are several companies that produce weapons. It's nothing huge, just an interesting touch that adds something for the more obsessive player to do... although, some of the sidequests would surely provide just as much of that.
Romances and party subplots are a little thin on the ground - but then when you take into account the fact that the game is so short, that's not surprising but more on that later - and on the first play through, despite trying to pursue a relationship with the blue chick, randomly the human girl ended up jumping Sheppard's bones... with no real warning... so, it was rather... hollow, not at all like the romance options in KOTOR or Baldur's Gate 2 where there could be lengthy and sometimes tricky romances. The sex scene that Fox News decided to start foaming at the mouth at is laughable - you're likely to see raunchier things in shampoo commercials. All-in-all, this aspect felt somewhat tacked on... a mere concession to the Bioware tradition. It's not bad but it feels so woefully underdeveloped and for one of the love interests to throw herself at you when you've not really pursued any of the romantic interest options is just poor.
Speaking of tacked on there are two other aspects of the game which feel rather incongruous in this way... the most obvious is the Mako. Basically, when you go to a planet that isn't in the main plot - you get dropped in the Mako... basically a magical tank that never gets flipped over and can go up almost vertical inclines. It's not bad... in fact, sometimes driving around and blowing up enemies in this fashion is actually pretty fun but sometimes bumping around on the uneven terrain of a planet, hoping to find stuff - ore deposits only show up on your radar, not the overview map. It's really the driving around on the uninteresting planets going from point A to B over abysmal terrain that proves to be irksome. In fact it feels as if they've tried to increase the action quotient with this aspect of the game as there are several main plot missions where you're essentially forced to hop in the Mako and drive along a preset route. It's just something that feels slightly out of place in the game but that for whatever reason Bioware shrugged and decided they'd stuck it in there, they might as well make use of it. It's not bad but it just gets boring after a while.
The other is the irritating subgame that you have to play EVERY time you want to try and open a locker or pick up an artifact or even survey minerals. Essentially you have to move an arrow to the centre of a circle in 15 seconds - although, hacking skills are still relevant and if you've gone someone that isn't imbued with the right skills, you should prepare for just about every box to be unhackable within relatively short order... not that it matters. Regardless, it's just a somewhat irritating and pointless endeavour that doesn't add anything to the game except a level of irritation. Fortunately you are sometimes afforded the chance to simply use "omni-gel".