Libraries/Frameworks
What libraries people use is very much a matter of preference. I was very fond of Prototype back in the day, but have made the switch, as so many others, to jQuery, since that seemed to be what was most desired by employers.
If you've got a project up and running that is based on some javascript framework, you want to make sure that the person you're hiring is proficient in that particular framework, regardless of whether or not it is "the best one" for the task, because you do not generally want to mix several javascript frameworks. If, on the other hand, you're a startup, you're more at liberty to go with whatever library your developer prefers.
Judging developer skill by library preference would be a real challenge. On one hand, jQuery is quite the new library on the block, so an impressive jQuery proficiency would be a sign that the developer has had the will and ability to quickly adapt to a new framework. On the other hand, jQuery is very easy to get started with, so a basic level understanding of jQuery doesn't really mean anything, and I've seen a lot of people who can use jQuery, but with no real understanding on how plain javascript works. I would really advise against drawing conclusions based on library preference.
IDE
As others have pointed out, it's hard to find an excellent javascript IDE. I'm from the .NET world so Visual Studio is the obvious IDE choice for me, since it's really well adapted to .NET programming needs. You'll find that a lot of really good javascript developers are not only javascript developers, and as such, their choice of IDE might not be based purely on what's good to write javascript in.
For what it's worth, Visual Studio 2010 has some real innovative intellisense for javascript. It has native intellisense for jQuery, and it also understands dynamically created variables (if you write for(var i = 0; i < 10; i++) window['myVar'+i] = i;, and later in your code starts typing myV..., intellisense will pop up suggestions myVar1, myVar2, etc...). VS2010 is only a release candidate, though, and Visual Studio 2008, which is the latest final release is terrible when it comes to javascript.
I would see a developer running release candidates as something positive, though =)
Tools
It cannot be stressed enough how much javascript development is facilitated by Firebug for Firefox. I definitely think that the developer should mention Firebug or Chrome developer tools, for debugging purposes.
Another tool it'd be great if the developer used would be PageSpeed for Firefox, which is an excellent way of inspecting what javascript functions can be deferred until after page load, for instance. This one definitely isn't a deal breaker, though.
I think the developer should be able to explain what JS Lint is, although there's no need for a proficient javascript developer to use it in everyday development, so it's not alarming if the tool isn't mentioned; it's just a good thing if the interviewee can explain what it is, if you actively ask about it.
As for minifying, I don't think that this should be a task that js developers should have to worry about. Minifying and script combining should be an automated deployment task, or an on the fly serverside task.