From the Statistics Directory
The descriptive statistics functions Mean, Median, Quantile, Variance, and StandardDeviation have been implemented in the kernel for operations on lists and matrices of data. This means that the statistics packages DiscreteDistributions.m or MultiDiscreteDistributions.m no longer need to be loaded to use these functions for data. The syntax otherwise stays the same as the package functions. The implementations for distributions still remain in the Statistics` packages, as do the other data functions from these packages.
Nonlinear curve fitting is now handled in the kernel via the function FindFit. The function NonlinearFit still exists in the NonlinearFit.m package for compatability, but it is just a shell for FindFit. FindFit should instead be used directly, as NonlinearFit will be removed completely at some point in the future. Note that there are a number of syntactic differences between FindFit and NonlinearFit, most significant of which are a different order for the parameter and variable arguments, and the return value consisting of a list of rules (in common with other numerics functions) instead of the model with parameters substituted. See the main documentation for more details.