No army was well-prepared when World War I began. Nobody seemed to have a correct picture of how that war would look like. The world hadn't seen much major conventional warfare. Europeans were accustomed to comparably easy killing of native people who had no modern weapons - far, far away.
Two wars provided the most relevant experiences:
1) Russo-Japanese war 1904/05
At land this war had experienced slightly outdated firepower that failed prevent the capture of fortified places by the Japanese.
Naval warfare included the effective use of torpedo boats, mines and a conventional blue water battle of battleships and cruisers. The latter confirmed the expectations about increasing effective gunfire range at sea.
2) Second Boer war 1899/1902
This taught a lot about infantry tactics; marksmanship at long ranges and so-called open order tactics to reduce infantry losses were in the focus. It did also provide some interesting (and drastic) guerilla/counter-guerilla lessons.
Other conflicts of interest included the anti-Boxer expedition 1900 (interesting because of its logistical challenges especially for countries like Germany) and some Italian warfare at about 1911/12 (first use of aircrafts heavier than air for war).
But that wasn't enough to understand modern, major conventional warfare. It took about two years of full-scale warfare (1914-1916) to draw the basic lessons and almost two more years to apply them.
World War I was shaped by railroad mobility, extreme firepower, high force densities and total war industrial mobilization.
The technology advanced after World War I, but many military establishments were again clueless about modern warfare without major conventional warfare experiences in the inter-war years (the Spanish civil war wasn't typical).
World War II was shaped by internal combustion engine-driven road and off-road mobility, really effective air power and radio communications.
WW2 still serves for many people as the base for major conventional warfare, but it's likely untypical for modern conventional warfare as were the 19th century conflicts for World War One.
The Israeli wars, Vietnam war, Falklands war and the latest two wars against Iraq showed only small aspects of modern conventional warfare as well.
These wars might be as poor a guide as were the Boer and Russo-Japanese wars before World War One.
World War III was luckily cancelled in time.
What might shape World War IV? What might we have missed in our preparations?
Remember; conventional major war is the only category of war (besides nuclear war) that can really take away our freedom and sovereignty! Great power gaming and kicking some poor paramilitary troops' asses in remote places is not really defence - its offence. The major conventional warfare capability is the justification for our armies!
Back2topic: My guesses about what would shape modern conventional warfare today are:
- sensor effects (relevant for offence and defence)
- electronic warfare (much more than ever seen before; many radio/radar capabilities might be diminished)
- hard-kill defences against ammunitions (heavily dependent on sensor effects, but probably the decisive push in the defence vs. offence struggle)
- an inability to protect overt war-relevant infrastructure/economic power against destruction (not applicable to all nations, of course)
Our ancestors of 1913 weren't dumb; it's apparently impossible to really know the nature of modern warfare based on theory alone. Some interested civilians offered better predictions before World War I than the General headquarters did - but almost all were completely wrong, just like almost all "experts".
The complexity has increased in the past century and we didn't have a complete demonstration of modern conventional warfare since 1945.
It's a safe bet to assume that many 'modern' armies today are as clueless about the nature of modern war as were the armies of 1913.
edit: I focused on technological aspects, as these are most easily to communicate and do coin some tactical aspects as well.