Complete Monster: Abaddon continues to live up to this, having entire planets destroyed in his wars.
Crack Is Cheaper: At launch it avoided this trope, especially if compared to the Warhammer parent games. The starter set came with 4 cruisers (the backbone of any fleet) for both Chaos & the Imperium, making it very easy to swap with someone on the "other side" to get 8 cruisers (which was almost as much as any fleet would need), while more plastic cruisers were cheap. With the addition of a metal battleship to act as fleet flagship, a light cruiser or two and 6 to 10 small escorts, you could have a fleet that would fight as large a battle as most people would want to play in a few hours, for a fraction of the cost of a Warhammer 40000 or Fantasy army.
The Orks & Eldar were more expensive as their entire fleets were in metal but the general fleet size of around a dozen ships would mitigate the total cost.
Fleets added later like the Necrons, Tyranids, Tau & Space Marines were harder to get a hold of, requiring shipping products directly from the Games Workshop HQ in the UK, which could push the cost right up for those outside the UK or Europe.
The Tau models that used their more typical aesthetic were Forgeworld only, and thus more expensive. The original version with boxy designs meant to represent their relatively low-tech space capabilities never really caught on.
With the models having ceased production prices are rising on the grey market beyond their original retail costs. Rare metal ships such as the metal battleships, Abaddon's Planet Killer, the Blackstone Fortress and Necrons can cost far more to acquire than originally.
Cult Classic: Pretty much from launch day. It was never going to draw the crowds the parent game did, but the miniatures were easy to put together & paint, the cost was cheap, and the game had simple rules. This ease of entry along with the overall theme of a space naval combat tabletop game being unique saw the game develop into a tabletop cult classic. Along with Mordheim, the game was popular enough to see a video game based upon it 15 years after the original tabletop game launched.
The Necrons. While most of the factions were reasonably well-balanced, the Necrons were obscenely overpowered in several different ways. They get the only armour saves in the game, their light cruisers are easily capable of going toe-to-toe with battleships of other factions, and their inertialess drive gives them incredible speed. Like their 40k equivalents of the day, the Necrons were overpowered by design, offset by a rule that granted more victory points than a ship's value to an opponent that managed to kill one, but it wasn't enough to compensate for the raw power the Necrons brought to the table.
Fighters/bombers/assault boats (and any ship that could carry them), at least initially. In the first iteration of the game's rules there were no limits on how many of these things you could have; as long as you had models for them, your fleet could basically continually pump out fighters out of their hangers every turn, which made escorts semi-worthless and turned the game into a contest of who could build up the largest cloud of fighters the fastest. A homebrew rule - which was later canonized in a later ruleset - made it so that the maximum number of starfighters on the board could not exceed the total number of launch bays in its parent fleet in order to address this issue.