We develop an analytic theory to estimate the glass transition temperature T-g of polymer melts as a function of the relative rigidities of the chain backbone and side groups, the monomer structure, pressure, and polymer mass. Our computations are based on an extension of the serniempirical Lindemann criterion of melting to locate T-g and on the use of the advanced mean field lattice cluster theory (LCT) for treating the themodynamics of systems containing structured monomer, semiflexible polymer chains. The Lindemann criterion is translated into a condition for T-g by expressing this relation in terms of the specific volume, and this free volume condition is used to calculate T-g from our thermodynamic theory. The mass dependence of T-g is compared to that of other characteristic temperatures of glass-formation. These additional characteristic temperatures are determined from the temperature variation of the LCT configurational entropy, in conjunction with the Adam-Gibbs model for long wavelength structural relaxation. Our theory explains generally observed trends in the variation of T-g with polymer microstructure, and we find that T-g can be tuned either upward or downward by increasing the length of the side chains, depending on the relative rigidities of the side groups and the chain backbone. The elucidation of the molecular origins of T-g in polymer liquids should be useful in designing and processing new synthetic materials and for understanding the dynamics and controlling the preservation of biological substances.