Consider the following sentences:

• What about the party didn't you like?
• Who from the film do you most want to meet?
• What about the story amused you?
• The news from Italy was the same.
• The results from the exams were published.
• That photograph of Mary doing a back-flip arrived.

These sentences all start with noun phrases. These particular noun phrases all have something in common. They all have a preposition phrase following the noun:

• What [about the party] didn't you like?
• Who [from the film] do you most want to meet?
• What [about the story] amused you?
• The news [from Italy] was the same.
• The results [from the exams] were published.
• That photograph [of Mary doing a back-flip] arrived.

Now these sentences are maybe a little bit clunky. Where these noun phrases are Subject, for instance, the preposition phrases mean that the head noun in the noun phrase is quite a long way away from the verb.

Lets look at the Subjects of these sentences:

• What about the party didn't [you] like?
• Who from the film do [you] most want to meet?
• [What about the story] amused you?
• [The news from Italy] was the same.
• [The results from the exams] were published.
• [That photograph of Mary doing a back-flip] arrived.

In the first two sentences, the noun phrases are Direct Objects of the verbs like and meet:

• [What about the party] didn't you like?
• [Who from the film] do you most want to meet?

Now one thing that tends to happen to preposition phrases inside noun phrases like these, is that they can break away from the noun phrase and appear at the end of the clause. This is sometimes called postposing. Sometimes writers call it extraposition from noun phrase movement. So in the following sentences part of the Subject noun phrase has broken off and moved to the end of the sentence:

• What amused you [about the story]?
• The news was the same [from Italy].
• The results were published [from the exams].
• The photograph arrived [of Mary doing a back-flip].

This doesn't only happen with Subject noun phrases. We can do it with the first two examples where the preposition phrases are part of a Direct Object:

• What didn't you like [about the party]?
• Who do you most want to meet [from the film]?

The Original Poster's example

What is unusual about Angkor Wat?

This sentence is a non-canonical version (one where the normal order of the phrases has been changed) of this sentence:

• What about Angkor Wat is unusual?

The Subject of the sentence is in brackets below:

• [What about Angkor Wat] is unusual?

In the Original Poster's version of the sentence, part of the Subject has been moved to the end of the sentence:

• [What] is unusual [about Angkor Wat]?

Note: The Italy example is from this book here

Syntactically every sentence has a complete subject and complete predicate.

The complete subject of the sentence in question is what about Angkor Wat.