Three outstanding features of String theory:
i) The number of dimensions is inherent into the theory. If you work in classical physics or quantum mechanics, the problem you are dealing with pretty much determines in how many dimensions you will write your equation. Say you are dealing with a particle moving along a straight line. A one-dimensional equation would be sufficed. If you are dealing with a particle moving along a surface or a body rotating about an axis, a two-dimensional equation might do the trick. In other words, you are plugging into the theory how many dimensions you need for a particular problem. Not so with String theory. It tells you plain and square that you must work in ten dimensions; otherwise the equations don't make sense.
2) In the Standard model, you need to plug in twenty parameters. One example is the ratio of the mass of a muon to the mass of an electron. These twenty parameters must be fixed, usually by some lab experiments. In String Theory you need one parameter, the length of the quantum strings. It has been conjectured that it is about the Planck size, about 10 to the exponent (-33), a decimal followed by 33 zeroes. If one would blow a proton up to the size of our sun, a quantum string would be no bigger than a baseball. This is so small that most likely it will be never measured. Nevertheless, a one-parameter theory would trump on any day a twenty-parameter theory.
3) In the twentieth century two grand theories evolved: the General Theory of Relativity in which gravity is shown as the warping of the space-time continuum; and Quantum Mechanics in which the other forces are revealed as interactions that exchange particles. In particular, for the electromagnetic force, the photons are carriers of the force; for the weak nuclear force, the W's and Z bosons; and for the force between quarks, the gluons. But gravity is the only force that stands outside of this scheme. To put gravity on an equal footing with the other forces, that is, have it as a force that would exchange particles, one would need a massless boson with spin 2. It was this particular feature of String Theory - that it does produce such a particle - that made physicists think seriously about String Theory as the theory that could unify all the forces in nature and hopefully give an explanation of the twenty parameters of the Standard Model.