Mass Effect had a really gripping story, full of mysteries and stuff to discover. It felt like you were on an important, but secret mission to save the world (cliche, but pulled off 11/11), and your choices really mattered. Also the world was really well made too - various interesting worlds and species that made sense, realistic political drama and all that. Combat was a bit repetitive and got boring quickly, especially if you didn't care enough to play on the more difficult settings, with your crew automatically using skills. Weapons and armour were all practically the same, just with slightly different stats. Overall it was one of those games you played for the story and immersive world.
Mass Effect 2 had much improved combat, more diverse weapons, armour and abilities. Bits of enemies could be shot off and the combat was just way more tactical and fun in general. The world was about as good as in ME1, except that you've already seen it all. But the story sucked - it was all about "collect your crew and win the boss fight". You didn't really discover anything new about the main plot, it was all known from the very beginning - bad aliens are coming and you have to stop them. The only "story" to discover was the less important back stories of your crew members. There was only a single story quest - save the world - available practically from the beginning. You only had to do side quests focused around your crew members, not around the actual main story.
