Mass Effect is a great game with an awesome storyline, good gameplay, great world, good characters, etc. It does have a few glaring flaws, however. When I say be careful, I mean be careful not to let the flaws turn you off the series. Mass Effect 2 eliminates most of the problems of its predecessor.

The main complaints I've seen:

1) Side-planets. Mass Effect 1 has a pile of solar systems you can visit, with several planets in each, most of which you can visit. There are resources to be collected, and sidequests to do. Here's the problem: EVERY PLANET IS THE SAME. Different terrain texture slapped on (sometimes), but it's all the same. A couple resource nodes to drive around and get, maybe a building composed of four generic hallways with a few guys in it. Always the same.

2) Loading times. When slowly progressing from area to area (in missions, fighting your way through) they're fine. When trying to run about in the main city, doing detective work for sidequests, they are painful. If you can play the game for the PC they're not so bad, but they're pretty awful on the 360.

My recommendation is to choose a low difficulty level (easy, or normal) and just play through the storyline. Take a sidequest or two if it seems particularly interesting or important, but ignore the random distress calls you get from planets, fedex quests in the city, etc. Just play through, enjoy the storyline, and then get ME2, which does great things with sidequests/exploration, and improves loading times ten-fold. Trust me and the rest of the people who have played ME1 when we say, you won't miss much.