Build 252 is a preparatory release that lays much of the groundwork for Reinforced, which will arrive this Friday. It contains big changes to basically everything. There will be no changelog at this time,* and the changelog will come with Build 253 (Reinforced) on Friday.
Lots of stuff has changed under the hood - You will notice for example that you may select from D3D9, D3D11, and OpenGL in the graphics menu. These new graphics-library options are indicative of the imminent Reinforced Linux release.
The game should be much improved in 252, though there will be some issues. You might get stuck occasionally, as changes to the physics system (again, in preparation for Linux) have introduced some new issues. Shotguns are also currently hovering in midair, and Grenade Launcher animations are a little bit... Eh.
And, there is a new icon for NS2! Enjoy, and get ready. Reinforced is coming - August 30.
Oh and one more thing - Enjoy the nades ;)
*Why no CHANGELOGZ? UWE iz teh nubcak3s!!!!! Answer - Creating changelogs takes time. Right now, everyone is totally flat out building Reinforced. The 252 changelog has fallen victim to triage.