C# emerged around the time that Java IDEs emerged. Basically, MS did a decent job with the C# language and tooling. However, it was very much inspired by Java, Java IDEs and Microsoft's own Java IDE (Visual J++).
I think IBM was first with supporting refactorings for Java in Visual Age and Eclipse. Refactorings themselves emerged out of the Smalltalk community.
MS was very motivated to support that for C# as they were getting worried about losing marketshare to Java at the time and because J++ got a lot of negative press (compatibiltiy issues, vendor lockin, the usual MS stuff). In the end C# was held back by the same factors that held back J++ and it took them until very recently to openly support it on platforms other than Windows.
Why C# exists isn’t very relevant to that, Microsoft actually made the IDE team a first class member st the table during C#’s design and evolution. Such and such feature thought to be difficult to tool even if it can compile? Well, fix it or cut it.
Microsoft intellisense (aka code completion) was truly a first back in 1997 (then just for VB and C++).
If that was the case for C#/.NET, they really didn't have much to show for it at time of release. IntelliJ IDEA that came out a year later was a fancier, more 'integrated' IDE.
But beside smalltalk, there were lots of others. All the commercial Lisps, Dylan etc. Somewhat more mainstream-ish - the NeXT Objective C environment is a good and much earlier example.
There were lots of interesting things about .NET when it came out, the IDE-ness of it doesn't strike me as one of them at all.
Yes they were. Delphi and Visual Basic are the most obvious examples of IDE-centric languages. You couldn't even buy the compiler separately from the IDE.
It's a fantastic initiative -- in such a short time we see many IDEs support consistent auto-complete, refactoring and so much more.