What is usually referred to as Lusztig's Conjecture in the modular representation theory of semisimple algebraic groups has been enormously influential, as seen in Jantzen's treatise Representations of Algebraic Groups. It is actually a series of closely related conjectures, from 1979 on, inspired by the (soon proved) Kazhdan-Lusztig Conjecture (1979) on the formal characters of the usually infinite dimensional simple highest weight modules for a complex semisimple Lie algebra: such a character can be written as a $\mathbb{Z}$-linear combination of the known formal characters of Verma modules whose coefficients are values at 1 of Kazhdan-Lusztig polynomials for the Iwahori-Hecke algebra of the Weyl group $W$. The original characteristic $p$ conjecture has a similar flavor, but with the affine Weyl group (whose translations are multiplied by $p$) replacing $W$ and with the essential proviso that $p$ be not too small. It is expected that the Coxeter number of $W$ will be a suitable lower bound, but so far the partial proofs by Andersen-Jantzen-Soergel, Fiebig, and Bezrukavnikov-Mirkovic do not achieve a reasonable bound.
If proved, the conjecture would combine with older results of Curtis and Steinberg to yield all modular irreducible characters of finite groups of Lie type in the defining characteristic (but still with the lower bound on $p$), as well as the formal characters and dimensions of all restricted representations of the Lie algebra of the given semisimple group. Andersen and others have formulated further consequences, in terms of the structure of Weyl modules, the extensions and cohomology of simple or Weyl modules, etc. (Adapted to general linear groups, there are also implications for modular characters of symmetric groups.) The later conjectures of Lusztig, proved for large enough $p$ in a preprint by Bezrukavnikov and Mirkovic, go further with the non-restricted Lie algebra representations as well in a unified geometric setting which promises further applications.
ADDED: I should point out that many special cases of the more general results which would follow from Lusztig's Conjecture have in fact been verified, but usually by computational or somewhat ad hoc methods. Plus the existing proofs of the conjecture itself for "large enough" primes, which don't seem improvable without new methods.