In mxml, you have no access to the constructor of the component. Therefore, you have no way of requiring constructor arguments to be set.
I was thinking, it would be nice to have a way to enforce at least that certain properties get set.

So here's my solution. Feel free to improve upon it, it's just the first way I could think of to do this.
First, we add a metadata tag to all required properties of your Component.

[Required] public var requiredArgument:Object;

Then, add this to your compiler arguments so the [Required] tag gets compiled.

-keep-as3-metadata+=Required

Download the following class to your classpath. (Preserve the packaging com/nbilyk/utils)
http://svn.nbilyk.com/public/Applications/NBLib/src/com/nbilyk/utils/Com...

Then in your component, add in the preinitialize event the following:

preinitialize="ComponentUtil.checkRequired(this);"

So your whole component should look something like this:

<?xml version="1.0" encoding="utf-8"?>
<mx:Canvas xmlns="*" xmlns:mx="http://www.adobe.com/2006/mxml" creationComplete="ComponentUtil.checkRequired(this);">
	<mx:Script>
		<![CDATA[
			import com.nbilyk.utils.ComponentUtil;
 
			[Required] public var requiredString:String;
			[Required] public var requiredNumber:Number;
			[Required] [Bindable] public var requiredObject:Object;
 
			[Required]
			public function get requiredProp():String {}
			public function set requiredProp(value:String):void {}
		]]>
	</mx:Script>
</mx:Canvas>

Note that int, uint, and Boolean types cannot be required as they will always have a default value.