On December 7th, 1941, the IJN was probably the most well drilled, and cohesive navy in the world.

They had taken a leap no other nation had, the concentration of carriers into permanent multi ship formations. Instead of temporary ad hoc comings together out of convenience, or a policy of isolated single carrier task forces.

That meant that the IJN could hit harder, and more often then the USN or RN. The Kido Butai, the main carrier force could utilize 2, 4, or all 6 of its large fleet carriers into an orchestrated ballet where each carrier could contribute one portion of its air wing to an attack and reserve the rest as needed.

Her surface fleet too was a real threat, from her destroyers up to the mighty battleships. Because the Washington and London Naval Treaties had limited Japan to fewer ships than either US or Britain, Japan sought to maximize the offensive power of every ship she built. many ships had what would be considered a dangerously over burdened weapons load by Western fleet, with even Japanese heavy cruisers carrying large torpedo launchers with the risks associated to them. The IJN was also very well drilled in night action, part of their gospel strategy was to attrite the USN by night surprise attacks by their light forces to be finished off by the battleships and carriers in the day. That meant that visual gunnery at night was a particular specialty of the IJN. The deployment of radar for search usage, let alone directing guns was slow though.

This was not a force without flaws though. In keeping with the other great fleets across the board her ships carried too light an AA armament, and some guns were not up to the challenges. While her carrier fleet was a polyglot mixture of ships of slightly different capabilities, only the new Shokaku class truly being the equal of the US Yorktown class.

While some problems were uniquely their own. The IJN had little to no interest in ASW or convoy work, making their vital merchant marine and trade lines vulnerable to the US submarine effort. While their merchant marine was too small to begin with and deeply struggled to keep the fleet supplied, the Empire advancing, and bring back the needed raw materials from the Southern Resource Area, let alone try to supply the fuel hungry battleships. Because of this the Yamato for instance spent most of her war in port either in Japan or at Truk, it simply was too costly to send her to sea when other ships needed the fuel more urgently, while Yamamoto could exercise strategic control from her with the help of shore based facilities as well. The IJN was also plagued start to finish with poor incorporation of technology from sufficient radios to radar, and by sub standard Damage Control practices, leading directly to increased losses. And arguably a supply of enough radios and an actual communications net similar to the USN could have averted such disaster at Midway.

All in all the IJN and Kido Butai was an incredibly powerful first strike and raiding weapon. It could bring heavy targeted blows quickly. But it lacked the depth and staying power to win a global conflict like the one it found itself in. It could even slightly recover from the loss of 4 carriers at Midway, but the bloody meat grinders of Guadalcanal and the Solomons ate up her reserves of ships, crews, planes, and precious air crews.

For an online intro I might recommend http://www.combinedfleet.com/kaigun.htm it is a sight managed by the two authors Parshall and Tully, the authors of Shattered Sword, THE modern work on the Battle of Midway.