The letter is here. Among other things, McConnell argues in some detail that conservatives will likely be pretty happy about Elena Kagan on the freedom of religious and political speech, on free exercise of religion, and on the participation of religious institutions in generally available funding programs. (McConnell is on the pro-Sherbert/Yoder wing of the conservative movement when it comes to free exercise, rather than Justice Scalia’s pro-Employment-Division-v.-Smith wing; but I suspect that these days many conservatives are with McConnell on this.) He also criticizes her position on military recruiters at Harvard, but praises her willingness to push the Harvard faculty to hire conservatives and not just liberals.
My sense is that McConnell is very highly regarded among conservatives, and especially social conservatives. (He’s also very highly regarded by me, but that’s not particularly relevant here.) His praise for Kagan thus seems to me to be especially significant.