Lately, there has been all kinds of research and books that have flooded the market about the plasticity of the brain. How you can indeed build new connections and synapses. One of the big ways to do this is to do something challenging where the brain really has to focus in. Some great examples of this are learning a new language or learning to play a musical instrument.
Physically I think there are some similar analogies that can be seen with learning new exercises. If you have been doing the old staples, (squats, deads, bench, dips, pull ups, push ups, curls, ect...) I believe your missing out. Don't get me wrong, they're old staples because they're great exercises, and should be foundational exercises of a great program. But, if that's all you are doing, you are missing out on some important attributes.
First, whenever you change something, even slight, such as your stance, you start to recruit muscles in a different way. This alone will lead to new soreness and growth. But that's not totally what I'm after.
Completely new exercises are what I'm talking about. Doing something that your first time trying you don't feel coordinated, you feel like you can't make your body do what you want it to do. Basically learning new movement patterns. I'm not talking about doing cleans off a swiss ball, where the danger out weighs the benefits, but trying out an exercise like a Reverse lunge with Valslide, or a TRX push up with one foot elevated.
As you learn to master the new exercise, your body creates muscle memory, your moving and stretching the fascia in new ways, your creating in my opinion a more athletic body. You're learning to coordinate new muscles, the mind body connection is further strengthened, and with all new research on fascia, potentially a list of other beneficial things.
So if you haven't felt a little foolish physically in awhile, pick a few new exercises that once learned will be a helpful supplemental lift for the old staples, your athletic ability will thank you.