What we know conclusively after three of these expansions is that none of them are ever “minor” no matter how small their list of new features and tweaks. Practically every patch these days winds up breaking or fixing or re-breaking The Game in interesting ways that are never readily apparent but only appear after long hours of carefully probing the depths of the mechanics. Heir to the Throne, which appears to do little, actually does a great deal and makes Europa Universalis III even subtler a game than it already was.
It also means that these reviews become increasingly incomprehensible to anyone not already well versed in the specific vocabulary of Paradox. If this is you, then try the In Nomine review first; Heir to the Throne assumes you own all of the other expansions already, so I think it's fair to assume knowledge of them here.
Anyway, the official list of new features is not on the scale of the list that accompanied last year's In Nomine, but it is substantial. The list includes a new casus belli system, alleged rebalancing between East and West to try and close the technology gap, an expanded Holy Roman Empire, new events, magistrates, cultural tradition, etc.
"The casus belli system is the biggest leap forward."
The casus belli system is the biggest leap forward. Wars are now waged either with a purpose or at great cost. Declaring war for no reason now results in significant stability hits and massive diplomatic problems. It's extremely ill advised. On the other end of the spectrum, declaring war with a legitimate reason (to reclaim lost provinces, to destroy the infidel) offers so many benefits that frequently you find yourself waging wars driven by nothing more than the possibility of a “cheap” victory.
The system changes two things about wars: the penalties for starting them and the rewards for finishing them. In starting wars, the system gives you incentives if you have claims on provinces belonging to someone else, if your neighbor is of a different religion, if he's kicked you out of his markets, or if he has been dishonorable in his relations with other countries. Should you finish wars like these, the peace settlement also benefits from a shrewd choice of casus belli. The penalty for acquiring provinces taken in an “unjust” war (one without a casus belli) is severe, but it's much easier to bear if you've been fighting a holy war, or if you had a claim on the province beforehand.
Of course, this system works both ways, so if the AI makes a string of terrible decisions and winds up being perceived as dishonorable, you can step in, smash them, and demand provinces or tribute without causing too much of a stir. Casus belli isn't just one more thing to worry about – it makes sense, allows more interesting things to happen, and gives a moral dimension to your skills as a ruler that wasn't there before.
The other stuff is less important. Allegedly, the ping-pong combat was overhauled, and armies are now supposed to surrender when the fight is well and truly lost. I did not observe this in my game. Magistrates, a new “person resource” (alongside merchants, colonists, etc.) have been added. They allow new, powerful provincial decisions to be made and help make your nation more efficient. The Holy Roman Emperor can now force member states to convert or annex them into a new Holy Roman Empire state, if he's got enough of the new HRE-specific Imperial Authority. There's also a little bit of Crusader Kings running around on the dynasty screen: your dynasty has “legitimacy,” a measure of the perceived strength of your family's claim to the throne. The actual namesake of the expansion pack, the heir himself, is also now a part of your deliberations. Having a robust heir to your throne is key to maintaining legitimacy and keeping the consequences of a low rating – revolts, claim wars, etc. – from occurring. The heir is randomly generated and his stats cannot be altered, as they could in Crusader Kings, but it isn't that big of a deal. It adds a sense of existential dread to running an empire: with low legitimacy, no heir, claims on your throne by powerful neighbors, and an aging monarch, you quickly become paranoid, and not without reason; everybody actually is out to get you.
The further on we get into the Europa Universalis series the more glaring its remaining flaws become. The flaw that is the most obvious and is still basically unremedied by Heir to the Throne is that the game becomes less and less entertaining the further away you get from Europe. Not only are non-European nations inexplicably less technologically advanced, many of them aren't even accurate in the most basic terms (Ethiopia is still a tribal despotism, for example). They still fall far behind Europe quickly, and very few ever catch up. This means that global imperialism of the 19th century model happens in-game several centuries ahead of schedule. Paradox doesn't seem to give a damn about countries where they have few fans, which is understandable but increasingly unacceptable.
Aside from that unpleasantness Heir to the Throne is essential, and not just because it's going to be required by the next expansion pack. It improves on a great formula, and the only areas it is lacking in are areas in which no other grand strategy game has ever excelled. Europa Universalis is now at the edge of truly groundbreaking innovation. Improving on something this polished is going to require entirely new ways of thinking about these kinds of strategy games.