Source 2 is mostly an optimization update. Those of us who've been using Hammer for a while will notice that the new Source 2 tool-set has (among other things) been massively optimized, especially in the way of file/folder directory set-up as well as the tools in general. It also takes advantage of DX 11, allowing for higher performance (and a better looking game in general). For those with 64 bit systems, it also really is useful due to the support of multi-core usage [finally!] (rendering times reduces, compile times reduced, etc.). It's essentially the same way Source is to Gldsrc (Half-Life 1 engine), except more subtle, since the graphical quality isn't changed as much.