I don't know of any .NET/Java programmers who say they can't live without IntelliSense. I certainly know I can, and do, frequently. Why, just this morning, I wrote some Java in Emacs. No IntelliSense there.
As far as async I/O, that is a problem. It might get better with the new GC, but proper async I/O would need to use one of the async I/O implementations available (libev/libevent, libaio, etc.).
I'm primarily a .NET developer on Linux. While I'm comfortable with other languages, and am quite proficient at using vim & and a little emacs - it simply makes no sense to not use an IDE for Java or C#.
These are tools built around the language and the workflow, and thus have been extremely tailored to ensure that productivity is high. While you can type the code out in any editor, and maybe even get CTAGS based completion and jump-to-type support, I haven't seen as tight an experience as MonoDevelop or Netbeans (or VS, but I don't know that very well).
There exists a tradeoff - you won't get the extensibility of Emacs or the efficiency of vi (they have vi-modes but it doesn't come close). Ultimately, when it comes to simply getting the idea from your head into the code, IDEs do help a lot.
As far as async I/O, that is a problem. It might get better with the new GC, but proper async I/O would need to use one of the async I/O implementations available (libev/libevent, libaio, etc.).