The ‘Mayfly December romance’ is the name given to the romance between two people with vastly different life-expectancies – the classic immortal/mortal relationship. There are two ways this can be handled in fiction – either it’s glossed over, or the discrepancy will be acknowledged and constitute the driving force of the plot. (I know it doesn’t sound like much of a statement, but there’s a point to be made there. It’s not a matter of ‘either they mention it or they don’t’ – it’s more ‘either it’s a driving force of the plot, or it’s not mentioned’.)
The reason that it’s so often the driving force of a plot is simple and straight-forward. When one of the parties in a relationship is immortal, or simply extremely long-lived, and the other party has a ‘normal’ or short lifespan… one of them is going to die, well before the other. In the case of an immortal, a long time before the other. All sorts of questions can spring from this – is one of them going to stay the same, while the other grows old? When one of them dies, what is the other going to do next? Is it worth getting into a relationship with someone who will only die in fifty, or a hundred years, when you know you are going to be here for the next thousand? Why suffer that heartbreak?
It gets more complicated if they have children together. Never mind the matter of how two different species can reproduce, who are the children going to have? Usually, they’ll have a lifespan of somewhere between the two parents, and unlike their parents (who could have pursued a relationship within their own species, had they wanted to do so), there’s no one on earth (or on Azeroth, or Middle-Earth) with the same lifespan as them. They’re doomed to being one or the other in a Mayfly December romance – would you want that, for your child, when you can see how hard it is?
You can see, it can be great romantic conflict fodder.
It’s a very popular trope, to the point where virtually any setting that mixes mortal and immortal characters will want to play on this – Lord of the Rings is one of the most famous modern examples, and use has continued into very recent years. Even the plot of Twilight is driven by the Mayfly December romance between a human and a vampire. So, of course you can see where I’m going with this – and why not – let’s add World of Warcraft to the list.
I’ve spoken before about interracial romance on World of Warcraft, and if you’re planning on pursuing that, I think this is a great place to start when mining for conflict in your role-play. Consider that while the night elves and draenei are extremely long-lived, the same is not true for humans, gnomes and dwarves (though I think you have plenty of problems already if you’re thinking about draenei/gnome romances). The same can be said for the different lifespans of the Horde races – though I repeat my comment about the gnome/draenei for virtually any Horde interracial pairing.
If you don’t like the idea of mixing the races (are they just too different for your liking?), or you want to dabble in the trope without getting too dramatic, there’s also the very real possibility of what the TV Tropes Wiki calls the May-September romance. This is when two people with a similar life-expectancy, but who are at different stages of their life, begin a relationship. Of course, we see this in real life all the time – there’s a long history of student-teacher crushes (and occasionally, ill-advised relationships), older men with younger women, and so on.
In this situation, some of the same aspects still apply – if you’re in a relationship with someone twenty years your senior, it’s a fair bet that they are going to predecease you – but in a less dramatic fashion. There’s no longer the prospect of living out an eternity without your true love, or your children being forced to suffer the same fate – it’s all just in the moment, and about a handful of years. There’s still plenty of conflict to mine here, in the possibility of being predeceased, and in public opinion. After all, when we see a balding, greying man, with a young, attractive woman, do we react the same way to seeing a balding, greying man and a greying woman, or two young, attractive people? Of course, we don’t. People wonder about money, about midlife crises, about daddy issues and cradle-robbing. Not everyone, perhaps, but people in general are mean and judgmental. Why would that necessarily be any different on Azeroth?
(Note: A large age gap in married couples is no new thing, and the cultural context does have quite a large impact. However, in lieu of a definitive answer on this subject, I’m assuming that Warcraft races tend to get married to people of a similar age to themselves.)