When a tree falls in a forest in Skyrim, it only falls for you. When a tree falls in a forest in SL, it falls for everyone withing rendering range, and might even make a sound. It falls in the servers, and the servers then tell the viewers of everyone around that it's falling. It's the uncertainty of the way physics work in SL (a desirable trait) that requires they be a server-side effect, so that everyone sees the same thing. The latency involved in this process is what gives falling SL trees, or anvils, or Karmann Ghias, their charm.
In Skyrim, when the tree falls, everything is optimized and happens locally, without the requirement of networked information zipping back and forth. The latency is very low, things happen very smoothly and seamlessly.