This paper analyzes the impact of feedback delay on the performance of multiuser selection diversity systems employing orthogonal space-time block coding (OSTBC) and rate adaptive modulation over time-varying MIMO Rayleigh fading channels. Rate-adaptive M-QAM modulation along with a user selection diversity is considered in this paper. Analytical expressions are derived for the average spectral efficiency, average bit error rate (BER), and bit error outage (BEO) probability of the system. Using the results obtained from the closed-form analytical expressions, as well as validation from Monte-Carlo simulations, the impact of feedback delay on the system performance is evaluated and discussed.