When you're doing any exercise you want to be establishing "muscle feel." Or whatever you want to call it. Whatever muscles you're supposed to be working, you want to feel them working! If you can't establish that feel, you're most likely using too much weight and should try lessening it. If you're aim is to improve your physique, it's all about technique and establishing muscle feel vs. how much sheer weight you're lifting. I think a lot of people start having ego trips and are more concerned about how much they're lifting as if anyone else other than them really cares.
If you're into bodybuilding, it's all about training the muscles. When you're weight training, you're using weights to train your muscles! When you're lifting, try thinking about what your muscles are doing rather than what the weight is doing. For instance, if I'm doing barbell curls, I'm focusing on my biceps working through a full range of motion, a wide arc and getting a good squeeze at the top, rather than focusing on the bar itself. Use the weights as a means to an end rather than the actual goal (unless you're into weight lifting, where maximum weight is the goal).
It also helps to flex the muscles you're working after your sets. This not only helps bring more blood into that area which helps you keep intensity high and helps you better establish muscle feel (when you're muscles are big and full it's easier to keep them tight and focus on them when you're lifting), but it helps you better learn your body. When you're one with your body you can better manipulate it during exercise to get as much out of that lift as possible. Everyone is mostly the same but there are enough differences in our physiques that we have to use some trail and error to tweak movements and techniques to suit ourselves and get as much of an advantage as possible.
So yeah, concentration, technique and establishing feel is VERY important!