Nothing at all. It's shit.

But one security model is better than having two security models.

If the components are sufficiently isolated, I'd much rather have many smaller components (with individual security models) than one monolithic system.

That's why e.g. the Unity3d plugin is so great. A complete 3D engine (with state of the art graphics, the ability to load almost any 3d format on earth, ...) does not belong into the browser. I don't want that code on my work PC.

OTOH, its great to have the ability to run games in the browser. The performance is much better than WebGL (because it contains compiled code, and because it is a complete, tuned, engine and not just an opengl implementation). And it's fairly safe, since it's built with Mono/.NET, you don't have problems with shellcode injection etc..

So.. stay with the shit model forever? Good idea! NaCl is part of an effort to fix all this bullshit, and it's at least sort of going in the right direction.