The concept of Galilean relativity (also known as Galilean invariance or occasionally Newtonian relativity) was already well established, and fundamentally stated that physical mechanics in all inertial (non-accelerated) reference frames in indistinguishable, regardless of the speed they are moving with respect to some other fixed frame. Einstein’s particular genius was to recognize that the is no privileged or fixed frames; all non-inertial frames are in constant relative motion to each other, and the behavior an observer in one frame sees in another is mirrored by that seen by an observer in the second frame looking at the first. The principle of a maximum speed at which mass and energy can travel falls out from this as a consequence. Einstein wasn’t the first to cotton onto this particular relationship; in particularly, Henri Poincaré developed a theory that was essentially identical to special relativity in many of its particulars (especially the E=m*c[sup]2[/sup] mass-energy relationship that has become characterized as “Einstein’s Equation” in the public mind) but did not publish his full theory until after Einstein published his third and fourth Annus Mirabilis papers in 1905, “On the Electrodynamics of Moving Bodies”, his introductory work on SR, and “Does the Inertia of a Body Depend Upon Its Energy Content?” Heinrick Lorentz had already developed the algebraic transformations that bear his name which both Einstein and Poincaré used in developing the spacetime relationships.
Poincaré continued to attempt to justify a version of a static aether frame, whereas Einstein dismissed the need for a medium and allowed for electromagnetic radiation to be self-propagating, though one can argue that this is nothing more than semantics, and that SR simply ignores the properties of the underlying plenum (something that can’t be done with general relativity). Hermann Minkowski, whose eponymous space describes a four dimensional manifold (three space and one time dimension) that distinguishes it from Euclidian space, developed his tool as an attempt to generalize Einstein’s (and Poincaré’s) theories, but did not precede them. It is true that Einstein’s papers don’t reference anything else (and while it is doubtful he knew about the work of Poincaré, he did directly draw from Lorentz) but the standard for citation on older papers is significantly more lax than it is today.
There are some specious arguments that Einstein’s first wife, Mileva Marić, who was also a student in physics and mathematics at Zurich Polytechnic, may have significantly contributed to Einstein’s famous papers, but aside from the fact that he turned his Nobel prize money over to her in exchange for a quitclaim (with which she bought an apartment building and failed to do anything remarkable in the fields of physics or mathematics) there is no evidence that her contributions extended beyond nerdy pillow talk.
As far as general relativity, it is certainly true that Einstein was not sufficiently skilled in the then fairly obscure and nascent area of differential geometry to have developed the theory on his own. In particular, mathematician Tullio Levi-Civita directly encouraged Einstein to work on incorporating gravity into relativity via the principle of general covariance, and while it isn’t clear how much interaction mathematician-turned-physicist David Hilbert had with Einstein (whose work inspired Hilbert to study physics), while Hilbert published his version of what became known as the Einstein Field Equations first, he fully credited Einstein with having developed the underlying theory. Most of the specific solutions to the EFE (which is non-linear and very, very difficult to solve except in topologically trivial conditions) have been solved by people other than Einstein, and in fact, most realistic applications can’t be solved in closed form or with the primitive numerical simulation tools available during Einstein’s lifetime.
The photoelectric effect is something of a grab bag, as many of the people who worked on the underlying theory didn’t even want to accept the consequences, leading, as they did, to quantized and dualistic behavior of fundamental particles. Study of what became known as the photoelectric effect (and underlying quantum mechanics) go back at least to Max Planck, whose blackbody radiation model, based upon the statistical mechanics developed by predecessors like Ludwig Boltzmann, James Maxwell Clerk, and J.J. Thompson, gave rise to the stochastic behavior of fundamental particles (though Planck never accepted this as being anything other than an approximation). Einstein certainly won the Nobel prize for his theory (largely because relativity was still too controversial for the Nobel Committee to award on that basis) but his work was, as nearly all theoretical science is, based upon the work of many notables before him. Einstein also did a fair amount of work with statistical mechanics; none of it was strictly revolutionary, but it did lead to the identification of novel states of matter.
Einstein’s fundamentally original ideas are that there is no privileged reference frames, that gravity is an artifact of the underlying curvature of spacetime, that matter is a bound form of energy, and that energy is quantized (comes in discrete packets). These may not have been totally unique to him, but he was the first to put them together into coherent, workable theories that were exercised by others. Does Einstein enjoy a stature in popular culture that is disproportionate in comparison to his contributions? Perhaps; certainly there are many other notable physicists who have made substantial, paradigm-altering contributions but who couldn’t be identified in a lineup by 99% of the population; that isn’t because Einstein unfairly capitalized upon these discoveries as being all his, but because he made such a memorable, idiosyncratic figure, not only in science but in international relations. (A quick perusal of famous quotations by Al finds more in regard to world peace than physics.)
As for the response cited by the o.p., I’ll just say that no figure is so great that someone won’t try to tear him or her down, and while there is reason to protest that there are many other physicists who have contributed to the overall body of knowledge that we currently enjoy in equal measure, arguing that Einstein was a “career plagiarist” is disingenuous and argumentative. He built his theories on the works and mathematical tools of others, and his successors did the same using his work. That’s the way of science.
Stranger