Last night, while reading one of my knitting books, I noticed that instead of making my knit stitches in the front leg of the previous stitch, I make them into the back loop. In other words, I make what is called a twisted knit stitch where I should be making regular ones. I've already finished two sweaters this way and am in the middle of making knitty's Cleo top right now, all wrong

.
What exactly is the difference in how the finished stitches look, depending on whether they're twisted or not? How can this influence a pattern?