Well, what can I say? There are a few things I would add to DeeTees's excellent post. As you might imagine, in longhaul there's a good deal more eating, drinking and newspaper-reading, a bit of sleep occasionally (crew numbers allowing), and you don't go home at the end of the outbound flight. Some of the en-route procedures are a bit more involved, and ATC in the more remote parts of the world can be, er, interesting! We also tend (at least on the B747, and in Virgin) to fly the aircraft manually a lot more than DeeTee's post would suggest he does.
Inevitably, and not extensively covered by DeeTee, there is a good deal of work going on before and during flight to anticipate and plan for problems such as engine or pressurisation failures. An engine failure, say in the middle of the Atlantic, requires some fairly complex decision-making about the (new?) destination and route of flight, as does a pressurisation failure over the Himalayas. Political factors may also need to be taken into account. For instance, would you divert into Pakistan on a flight destined for Delhi? This kind of stuff needs thinking about before it becomes an imperative!
Other, routine, problems might involve weather avoidance en-route. In most of Europe or the US this isn't really an issue, as radar cover is sufficient to cope with track deviations without too much hassle. However, in areas covered by purely procedural (i.e. non-radar) rules, and using HF (short-wave) communications, a track deviation may take an enormous amount of effort to organise and so must be requested long before it's needed.
Weather, equally, can have fairly profound effects on what happens at your destination. While the forecast might show the weather well within your limits and the capabilities of your aeroplane, it may be sufficiently bad to seriously disrupt less well-equipped arrivals. That will have knock-on effects to all arrivals, such as extended patterns, holding or even a diversion. An in-flight delay may well be easily absorbed (but will have a domino effect on your aircraft's subsequent departure), but a diversion of something as large and complex as a full 747 is a major undertaking, especially if the only suitable diversion field is not particularly well set up for the procedure. If you have to terminate the flight at the diversion field, the problems of handling, accommodating and victualling 500 or so unhappy passengers should not be underestimated!
So, it might be dull on occasions, but it always has the potential to become very interesting, very quickly! And the view is always outstanding. So, on occasion, are the down-route parties - only on multi-night stops, of course - but that's another story.