Continued evolution of digital technology is occurring at breakneck pace, so don't be tempted to consider a digital camera an "investment" in something of stable longterm value. There will always be something better released in a year and whatever camera it replaces will plunge in value. Leica M8 owners felt mighty grumpy when the M9 was released and for the first time, the older model's value tanked. You could dish up the schadenfreude with a spoon.
Good lenses, however, do hold their value.
There is plenty of room left for advances in low light, high-ISO performance, autofocus performance, enhanced dynamic range, reduced image noise, better display performance. I'm intentionally leaving off pixel counts--we need clean pixels far more than we need more pixels. I suppose higher image bit rates will be welcomed at the advanced end, but I understand that takes real hardware firepower (i.e., expensive).
Video is in its infancy, and a poor fit in dslrs, and is driving a lot of the technical advancements.
With all that said, what makes a good photo is 20% camera, 80% the person holding the camera. As long as a digital camera remains in good condition, it will take the same pictures five years down the road as it did when it came out of the box.
My "trap" is I start doing new types of shooting and hit the camera's limits, then want something better suited to that type of shooting. Naturally, when a better performing camera hits the market I'm tempted to jump. Marketers seem to know this. :-)
"Something I'm curious about: will there be further advances in digital, so that cameras bought now will be redundant in a couple of years, or is it now a mature technology?"