jQuery gives you access to crisp and concise functional programming methods. Since the release of method chaining in (LINQ) in C# 3.0 this works very well for .NET programmers. So the flow from one language to the next is easy. To be able to query the DOM for an object, or a list of objects, works much better for us. It is jQuery's selection power first that makes it so attractive, then the extendability of it, and of course all the built in features that come with it are nice. Also, the community behind is wonderful in that I first look to see if someone else did something and then attempt to do it myself if a solution was not found. And last...but certainly not least...the fact that Microsoft is going to include in in Visual Studio 10 and support it is great. Moo Tools, Prototype, etc. just can't compete with all of the above.
