If you've been working out and watching what you eat, you've no doubt heard about CICO, or calories in, calories out. If you want to lose weight, you need to intake less calories than you expend through the day. To do this while still getting all the nutrition you need, you have to minimize the empty calories. That's why low carb diets work so well.
But this post is not about nutrition as it is about the workout itself. Most people understand the concept of empty calories, but then waste reps at the gym. Swinging the weights, not getting the heart rate high enough on the elliptical, resting for way too long between sets, picking a weight that is too light, or even forgetting to breath during the exercise! All of these will not only stop you from getting to your fitness goals, but will also waste time that could be better used for the rest of life!
Personally, if my workouts last longer than 30 minutes, I'm doing something wrong. I'm a big fan of high intensity workouts that push me to the maximum. But high intensity doesn't always mean high heart rate, or big heavy weights. Even in a basic bicep curl, I'm squeezing my core, butt, and bicep as hard as I can during every rep, maximizing my own effort, and breathing as much as I can.
The best analogy I can come up with is if you're at work, and want to get a promotion. You work your butt off, turn in work that is above what is called for, and love the process to EARN the promotion. You don't get rewarded just by showing up.
It's the same with your workout. Every rep counts, just like every calorie, or work assignment. And if you can't push yourself to that point, that's where a good trainer/coach comes in handy.