Support sandboxing! Allow memory usage limits for scripts.
The CLR allows us to run scripts with high security (so that they don't have full access to the PC) and high performance at the same time. However, in a server context one may want to run a thousand scripts written by different people at the same time. Therefore, no single untrusted script should be allowed to use a large amount of memory. .NET should provide configurable memory allocation limits on a thread/fiber. CPU time quotas are needed too, although (unlike memory quotas) perhaps it's possible to enforce CPU time limits without support from the CLR itself.