This is when a new technology has turned warfare on its head, and everyone is experimenting. All sorts of weird and wonderful ideas are tried out.
1950-60s Jet Era is fun
This was what got me thinking about this topic. A visit to the nearby Caloundra Air Museum confirmed this as a favourite period. The early jet era was so darn interesting. Missiles could be wire guided, or heat seekers, or radar guided - heck sometimes they simply fired salvoes of dozens of unguided rockets in hope of hitting something. My favourite - an unguided nuclear rocket. That's right - the AIR-2 Genie was a 1.5kt warhead fired in the 'approximate direction' of enemy bombers. With a range of about 6 miles, it had a 300m lethal radius - although I bet it would cause anyone within miles to poop their pants! With awesome cannons you can fit your arm into, these early jets have such primitive controls - going 900kph in using controls not much different than a early-war Spitfire. It's such a fun mix of designs. The weird P-38-style twin booms, fighters with twin engines underwing like Meteors, F-104 pointy noses contrasting with gaping blunt Mig-15 air intakes. Delta wings, swept wings, straight wings. Sometimes crew sat behind each other - sometimes side by side. You want to fit in a radar? Simply add a metre of metal cylinder and stretch the nose. You can tell the designers were really making it up as they went along. Even the paint schemes are colourful - or just bare metal.
Modern Air Combat is dull
For the modern era (1980s-) aircraft are so boring by contrast. NATO and Russian aircraft look pretty similar. Most have two engines. Most have two tail fins. All have swept wings. Most carry a pretty standard loadout - a single gatling-style cannon, a pair of heat seekers and several radar guided missiles. Paint schemes are a subdued grey. The biggest difference tends to be the electronics/radar - the stuff that is under the hood anyways. (It's no coincidence, my favourite "modern" aircraft, the 70s-era A-10, rips up the rule book a fair bit!) Aircraft, no matter who makes them, are getting more and more alike. The technology is relatively mature, and hasn't changed a lot since the 70s-80s. (Yes, I guess there is "stealth" but it isn't that widespread, and all the stealth fighters are clones of each other as well).
Early Renaissance is fun
You have pike, musketeers, aquebusiers, crossbowmen, halberdiers, sword-and-buckler men, cannon in a range of shapes and sizes. Units are mixed in a range of ways and ratios. You have cavalry in an equally wide range of varieties - dragoons, reiters, hussars, cuirassiers, harquebusiers, etc. There is a glorious mish-mash of colourful and varied units. Armour is still a "thing" and units wear varying amounts and types. You can also go cool places - and fight foes as varied as Indians, Aztecs, Ottomans, or Africans.
Napoleonics is dull
In contrast, Napoleonics seem pretty boring. It's mostly the pretty uniforms. The technology and tactics of the muzzle-loading musket is quite "mature"and polished by now. Everything is so "standardised" in comparison to the early days of gunpoweder. You can get infantry, cavalry and cannon, and the main difference is the red or blue uniforms. For ACW, simply substitute the uniform colour to grey and blue. That's why historical gamers from that era tend to be fussy (aka anal-retentive) about uniform - ' cos it's the only interesting thing from that era!
Heck, even Napoleonics naval (much as I like sailing ships) is pretty samey. The French and British copied each other (and captured each others' ships) so often the flag was the main difference. Ships were mass produced to a 'type' i.e. a 74, or a 36-gun frigate - often directly copied from rivals.
(In contrast, the ACW-to-pre-dreadnought era of naval warfare, where armour and explosive shells were only just becoming widespread, and steam replaced sail, is weird, wacky and interesting - at least until warships became relatively "standardized" after WW1.)
Modern Combat is dull
As I've been painting some 28mm stuff, I've been realising it's actually pretty samey. Everyone has four man fire teams, with maybe a SAW and a guy with an underbarrel grenade launcher. Most nations have small arms and support weapons in similar calibre, capacity, and capability. Even armoured vehicles are pretty similar - a 6x6 with a HMG, a IFV with a chaingun, a MBT with a 120mm. Really, is there that big a difference between a Challenger 2, an Abrams and a Leopard 2? Enough to care? C'mon, they're functionally identical.
WW2 Combat is Interesting
Compare this to WW2 tank combat, where they were still experimenting with what made a good AFV. We have tank destroyers, assault guns, bren carriers, infantry tanks, scout tanks, heavy tanks, medium tanks. Half tracks, tracked and wheeled APCs. Was there a difference between a T-34, KV; a Grant, Matilda, or Sherman; and a StuG, Panther or Tiger? You bet. Massive change (and thus a massive variety of designs) occurred in the space of a few years.
Small arms were varied. SMGs, semi automatic and bolt actions were represented in various ratios instead of the ubiquitous assault rifle (though there were those too!) A Bren gun, a MG42 and a BAR are not at all alike; whereas a modern SAW - if it isn't a Minimi or direct copy thereof - is functionally identical. Even handguns were a mix of revolvers and automatics of different calibres, and not just all 9mm automatics.
Admittedly, I don't play WW2 very often as it's so over-represented. I'm about as excited by new platoon-level WW2 games as I am by near future sci fi (Vietnam in space) - or any game with zombies in it (i.e., not at all). However, it is an interesting period of warfare than modern combat; as so much change occurred, and forces are more varied.
The Argument:Transitional periods in warfare, where a relatively massive technological change has recently occurred (introduction of gunpowder, automatic weapons, the transition from sail to steam, from propellers to the jet engine) is far more interesting than the later stages of that technology, where everyone has figured out the "best"way to use it, and are all copying each other/using cookie-cutter equipment.