I'll try to response within the limits of the NDA, please excuse the potential vagueness of some of the responses.
As you guys guessed already the Client is in unity.
As for the server core, its basically a black box that handle all the low level infrastructure, so the Lua scripting side basically focuses on defining all the aspects related to gameplay, content, behavior and so on. Lua was chosen as its used in various games that allow modding, nothing indicates that the scripting language will change. Personally Lua is a bit of a salty language, not the most pleasant but not all that bad either.
Keep in mind the end goal of Shards Online will be to provide tools to empower modding for everyone, well as much one can do with tools of course. So while scripting is an important aspect its only one aspect of the bigger modding picture.
Beyond this point any more specific details might be a breach of stuff. However in my own experimentation I was able to extend the lua environment to provide file i/o functionality, http requests. Basically there all sorts of ways to bend the server to your needs. One other early admin had posted his experiments on injecting a C# interpreter into the server so they could use C# instead. Don't know what happened to that person as they haven't posted in ages.
Darklotus you certainly sound like you know your way around code so I am certain you would find all sorts of ways to bend the server to your will. The same goes for anyone with a certain amount of developer experience. So while Shards Online scripting is built on a model of expected use there is absolutely nothing restricting you to that.
If I remember correctly once the alpha phase begins (currently pre-alpha) server admin access will be open to all.