Einstein is probably one of the most abstract thinkers who has ever lived. I don't think he's overrated, it's just that a lot of his theories are hard to picture and fully understand. Much like quantum theory (And BTW, he was the first scientist to suggest the name quanta to describe the energy transfer between different molecular excitation states).
Einsteins "General Theory of Gravitation", is one of his most profound and yet least understood theories. Basically, it redefines gravity through acceleration and mass. Einstein pointed out the you can not tell the difference between gravity and acceleration in a closed system. He then goes on to explain the connections between acceleration, mass and inertia. He does this in a very elegant, logical, and verifiable way.
Einstein's theory basically binds the universe in a huge 'space/time' tapestry, with the limit being the speed of light (in a vacuum). This important limitation, the speed of light, gives different results than Newton in high-speed/ high-energy situations. It still works like Newton on the smaller/slower scale, but helps to better understand truly galactic events. And also explains small high-speed ones: Scientist noticed early on that certain short-lived, high-speed particles seemed to last longer than they were predicted to. Eventually, it was discovered that this was due to the time-dilation effect of their high speed, as predicted by Einstein. Once the equations were altered to account for the speed (as laid down by Einstein), they fit perfectly.
The other limiting factor in Einstein's theory was mass. Truly massive objects 'bend' the space/time fabric of the universe. Any object that travels through this 'bent' area has it's path likewise 'bent'.
Imagine a trampoline, with a heavy cannon ball near one side. Now imagine rolling a coin, on it's side, in various directions across the trampoline. If the coin rolls close enough to the cannon ball, it will follow the 'bent' fabric and will likewise be redirected, as it tries to follow the 'path of least resistance'. This helps explain both gravitational lensing, where the light of a galaxy is 'split' in 2-3 paths around another galaxy, and black holes, which are so massive that even light can't 'get across'.