Anti-Climax Boss: Once you get over the grandiosity of Heinrich I and realize how limited his effective range is, the fight becomes a laugh as you kite him around and run away from him when he causes you to gravitate toward him. The Venom Gatling Gun chews him up while panzerfaust rockets help soften him up. Even the Tesla Cannon is effective, hitting Heinrich and his undead simultaneously. And yes, he's listed as That One Boss boss too, that's YMMV for you.
Lopers are crazy fast and can take a lot of hits. It's almost impossible to kill one without losing at least a third of your health/armor.
Elite Guards can be incredibly frustrating to deal with. They're more competent than most guards, taking effective cover and dodge-rolling, and very accurate with their Sten. On top of that, each bullet hurts badly. Mercifully, they are only in a select few levels.
The Black Guard Paratroopers are close behind Elite Guards in deadliness, but not quite as bad as said Elite Guards. In the penultimate level, they also show up alongside the Elite Guards, making for a deadly combo.
The Sten. It's silenced, fully automatic, quite accurate, does more damage per bullet (10) than the Thompson (8) and the MP-40 (6), which uses the same type of 9mm ammo, said 9mm ammo is literally the most plentiful in the game, and some versions of the game give it a rate of fire faster than the MP40's. As long as it's fired in bursts to avoid the overheating drawback, it's one of the deadliest weapons against any kind of enemy save for the Lopers and the super soldiers and is your go-to stealth weapon along with the companion, extreme-range Snooper Rifle.
The Venom Gatling Gun is a close-range devastator, able to mow down a group of Nazis in seconds, and is reasonably accurate when fired in controlled bursts. 12.7 mm ammo is moderately frequent after the Secret Weapon Facility mission, and the capacity is a very generous 1500 total rounds. It takes the edge off of encounters with prototype & uber cyborgs, and chews up the Final Boss to boot.
Polished Port: The PlayStation 2 and Xbox ports of RTCW add a new prologue level set in Egypt that shows what B.J. and Agent One were doing before their arrival at Castle Wolfenstein, introduce new enemies and weapons, includes a Co-Op mode (with Agent One as the second player), and B.J. is fully-voiced for the first time in the series. The Xbox port also features Wolfenstein 3D as an unlockable extra that you obtain after completing the main game.
Scrappy Weapon: Grenades. Nazis easily run away from them. Even worse, they can kick them back at B.J. It takes B.J. a long time to charge them and throw them at long distances, and they easily bounce in wrong directions like rubber balls.
If you already have trouble fighting the Proto-Soldier, than you'll be in hell once you fight the Ubersoldat. The Tesla Gun prevents you from ever moving away to safety, draining your health away unless you're lucky enough to find a barrier, forcing you to take potshots. Even worse is that a dozen of enemy scientists will arrive taking potshots at you with their Lugers as you'll be desperate to run for armor and health. The mission's end cutscene even mentions that BJ is beaten pretty badly after the battle.
Heinrich I. Not only does he take a lot of abuse, cause a lot of damage with his sword (one strike is either a One-Hit Kill or takes away most of the armor and half the health), and have an army of zombie warriors protecting him, but he summons ghosts to smother the player should the player stay too far back. Touching these ghosts will darken the screen, slow you down, and take massive chunks out of your health all at the same time. Also, you aren't even given any Panzerfausts during this battle, so you better have saved them up from earlier.