Anticlimax Boss: When you finally meet him, Zymran is a pushover for your Terminators, your heavy weapons or even your Assault Marines. He has pretty much the same stats as an elite Khorne Berserker and uses the Power Sword instead of the more powerful Power Fist. Any one of his Chaos Terminator bodyguards are actually stronger than he is.
Both Greater Daemons as well. The avatars of dark gods of cunning and rage, monsters who can supposedly wipe out entire armies and devastate planets, fall in a single turn to your Terminators. Even moreso the Bloodthirstier due to the Boss Arena Idiocy (see the main page). The Lord of Change isn't very bright either, and will usually let the player have a full turn worth of shooting before it retaliates. On the other hand, one should also not forget that by the time you reach these daemons, your Terminators will likely have the experience rank of Mighty Hero, equal to a Captain in the tabletop game. So while 5 marines taking out a Bloodthirster is very, very unlikely, five Space Marine Captains in Terminator Armour - that's a different story.
Crowning Music of Awesome: The soundtrack is considered so good that some players of the tabletop game use it as background music even all these years after Chaos Gate was released. Sample music here.
Demonic Spiders: The Flamers. They're tough and fast, can set your guys on fire (meaning they take damage and lose turns until it goes out), cannot be set on fire themselves and, worst of all, they will almost certainly set you on fire if you try attacking them in close combat.
Game Breaker: Quickening. Gives the already deadly Assault Cannon even More Dakka. Has been known to wipe out entire squads in a single turn. Also doubles the number of blows your marine can land in close combat, making the Thunder Hammer bearing Terminators ridiculously powerful.