What is it about this receipe that makes it an IPA? Or more generally, what makes an IPA?

Pale Ale - mostly lighter grains and less darker, maltier grains. Then a certain amount of hops throughout boil for flavorful bitter IBUs to off-balance some of the malty sugars. You then ferment with specific American, English, or Irish yeast. That's a Pale.

Now, an IPA comes from the hoppy mouth feel and that comes from beautiful noted hop oils swirling around during the fermentation process and secondary. These oils were needed to "preserve" English Ales in the crazy heat of India during English colonization, hence India Pale Ale. The nice Ale one would enjoy in an English pub became an India Pale Ale via the method used to preserve and maintain some freshness by using hop oils that had a side-effect...hoppiness.

God Save the Queen!

You may also find this informative:

http://en.wikipedia.org/wiki/India_Pale_Ale

It seems IPAs are currently fashionable—these things go in cycles, don't they?—and almost any copper-coloured ale is likely to have an IPA label slapped on it. I was amused and mildly disgusted to see that Baron's, a maker of kits, has something they call East India Pale Ale. As if IPA was ever anything but...

The Beer Judge Certification Program (BJCP) makes an attempt to describe styles such as this. One such style is IPA (grouped into 3 subcategories: English IPA, American IPA, and Imperial IPA). Have a look: http://www.bjcp.org/2008styles/style14.php

You're right that the hop character defines the IPA style. However, a traditional IPA has some restrictions on the malt and yeast character: you'll generally have a pale beer with low levels of malty sweetness (with occasional hints of caramel, toast, etc.) You'll generally see neutral fermentation character, though English IPA's will have some fruity esters.

There are specialty categories that have emerged which deviate from this kind of malt bill. "Cascadian Dark Ale" or "Black IPA" add some roasted notes to the malt. There are some wheat-based IPA's on the market. Some IPA's are brewed with lager yeast. From a competition standpoint, these are generally recognized as specialty beers, though there is a push for some styles, such as Black IPA, to be officially distinguished.

It was a lucky guess. There is nothing in that grist bill specific to an IPA. AAMOF, it's NOT an IPA! The OG is too low, for one thing. The recipe even calls itself a pale ale, not an IPA.

As to what makes an IPA an IPA, the best ROT is the BU:GU ratio. An IPA will usually be in the 1.060-1.075 OG range and have at least a 1:1 BU:GU ratio.