emacsclient is about as lightweight as it gets, and Cocoa Emacs itself (24.1.50.1, built from the Git repository on Savannah this afternoon) starts up in less than 5 seconds on my Mac. As for "scripting in a number of different languages", that's more of an anti-feature for me. Specifically, running Emacs in a Windows, FreeBSD, Linux, Solaris, ... VM in VMware Fusion, I can symlink the init files and site-lisp directories in my guest OS home directory to the analogous items in my OS X home directory, and, given reasonably comparable fonts and Emacs versions, my Emacs configuration and most extensions work identically in the VMs.

If, on the other hand, these extensions were written in a number of scripting languages rather than Emacs Lisp, I'd need to maintain a like number of scripting language runtimes in each guest OS. Given the amount of code I write in Emacs pales in comparison to the amount of contributed third-party code I use, the fact that I have to write it in Emacs Lisp is a vanishingly small price to pay. Besides that, for lightweight scripting at least, I actually like Emacs Lisp.