My number one biggest desire in C#, or complaint (if you choose)... Are we ever going to see const methods, objects and properties? I'm one of those who basically think languages we should actually be delimiting non-const items, and having everything default to const, but even though there are work-arounds, not having a const specifier I think encourages people (like me) to be lazy and write really dangerous code!
Const is very hard to do a multi-language environment. You would need to do it as part of metadata, and you would either only do it in some languages (which would reduce its universality), or require every language to support it (not a good thing).
My personal experience convinces me that const is a good idea in the small, but doesn't work too well in the large. Whenever I tried it in C++, I'd always run into the case where a method that was const needed to not be const (and it wasn't a case where mutable would have helped). At that point, I either needed to de-constify all call paths down to that function, or split the api into const and non-const versions. The first is a lot of extra work, and the second will decrease my overall quality.