Ryu doesn't learn Irene's real name until the very end of the first game. However, the manual already spoils this fact. However, the original Japanese manual made her name a complete mystery.
The names of the four human bosses Ryu must face and then some are described in the manual for the first game.
The same thing also applies for said first game to the name of each of the stages.
Awesome, but Impractical: The spinning slash and fire shield in the first game. The first causes you to do a spinning slash every time you attack while jumping, which can deal obnoxious damage to bosses, even killing the first one in one hit if landed correctly. The problem is it causes you to do a spinning slash every time you attack while jumping, meaning you can't control how you use your special attack energy, and will probably run out of it (unless you know the trick: hold down while attacking). The fire shield makes you invincible for a little while, but the problem is that when it times out, you lose it and get nothing to replace it.
"Blind Idiot" Translation: Basaquer, Kelbeross, Malth and even Big BadJaquio fall pray to this in the original NES trilogy. Their actual names were supposed to be "Berserker," "Cerberus", "Mars" and "Devil King" (Jakiō.) The mistranslated names do have plenty of charm, though...
Check-Point Starvation: The first NES game was generally pretty good with checkpoints, as you would usually respawn at the same screen you died at...unless you died to a boss, in which case you're taken back to the beginning of the stage. To make matters worse, if you're unfortunate enough to die at any of the three final bosses, however, you're kicked all the way back to the start of 6-1 instead of 6-4.
Cutscene Incompetence: The first game contains one of the earliest examples ever of this trope. Despite being an elite ninja, Ryu is knocked out and captured by Irene Lew in a cutscene after the first level, and only gets out of prison after she lets him out. He later gets captured by CIA agents (the second time he's captured in a span of 3 levels) and forced to work for them. Eventually he is manipulated by a Hostage for MacGuffin situation in which he hands over the demon statues Jaquio to prevent him from killing Irene. Natrually, Jaquio takes the statues, doesn't release Irene, and dumps Ryu down a pit trap, forcing him to fight through long levels just to get back to Jaquio again.
Dual Wielding: Basaquer in the NES games dual-wields butterfly knives.
Hard Levels, Easy Bosses: The first NES Ninja Gaiden. Barbarian, Bomberhead and Basquer were all ridiculously easy once you got the pattern down, an easily-exploitable glitch could make Kelbeross a pushover, Bloody Malth is just a matter of getting close to him and mashing buttons and the Masked Devil just requires you to hit the giant orb in the middle. Jaquio, however, is ungodly hard, and the Demon is largely luck-based. The sequels evened it out quite a bit.
Love at First Sight: Both Ryu and Irene in the first NES game, after an entire adventure without any proper build up for romance. In fairness, Ryu took the fact he was able to meet her as a fitting payment for all the trouble they went through, and Irene seems to have the same mindset on this matter as she disregarded Foster's direct orders to kill him. The result is the couple kissing at the end; as of Dead or Alive, they are Happily Married and running their Antique Shop together.
Puzzle Boss: In the Boss battle against Ryu's father, trying to strike him will get you nowhere. To win the fight, you have to destroy the statue casting orbs of energy towards him. (Which isn't hard, once you catch on. Or if you watched the cutscene right before the fight, which shows you what to attack)
Single-Stroke Battle: The opening cutscene of the NES Ninja Gaiden, where Joe gets defeated via this. He later turns out to be alive...
Spin Attack: The Jump and Slash Technique in the first game is a powerful art which turned Ryu into a flying buzzsaw and had the potential to take out bosses with one good hit. It's no wonder why it was removed in the sequels.
Stupid Surrender: In a cutscene in the first game, Ryu surrenders to a few CIA agents who point guns at him... right after finishing a level in which he had to defeat numerous enemies, including several of them who were armed with guns, though, because with Dr. Smith dead and only a warning about the demon statues from him, Ryu didn't have anyplace else to go. He correctly surmised that going along with them would get him the information he needed.