I think extremely loose clothing is a no-no. The flapping clothing can get in the way, even if you are just using the treadmill. Also, trousers that don't drag the floor is important.
One thing I found out is that boot-cut or bell-bottom pats is not good for cycling. The flappy bottoms get in the way of free movement.
A good set of shoes is necessary if one is susceptible to heel spurs or the like. And if one has ankle problems, a higher-rise shoe is probably going to be more helpful for proper movement.
Something too tight is not good too, esp. for women. The tight trousers can injure the V during rigorous workouts. Breathable cotton is my favourite. 100% cotton is tough to find though, and often expensive.
Good support bra. Don't want premature sag and nobody wants to see your breasts flopping all over the place.
I think ankle and wrist weights are important to help maximize workouts.
Style doesn't matter to me when it comes to working out. Clean, durable, and utilitarian are what I look for when I choose something to wear during training. The clothing is going to be ruined by sweat and faded by repeated washing. I don't spend money on style for workout clothes.