Battlefleet Gothic was an awesome game; I still have my Grey Knight/Inquisition fleet proudly displayed, painted up on a shelf. Each fleet had a different design behind it, and honestly, I thought they worked really well, and most of them suited their fluff appropriately.
Imperial/Chaos (since technically they were all Imperial tech) were built like old Earth Navy... most of the guns were on the sides, and everything came down to monstrous broadsides. Chaos ships, being older and pre-heresy, were faster and had access to more advanced tech than the Imperials lost, so they had better carriers and had access to more Lance weaponry, which were direct fires that could cut through armor. Imperials, to compensate as they lost that tech, went in tried and true fashion; they bolted a shit ton more armor on and made huge, slow, ponderous ships that needed armies to beat upon. Then they added tons of torpedos that could be launched through tubes in all that armor plating. They also got Ordinance weapons, making them like IG in space.
Space Marines, to show the Codex at work, only got Assault Cruisers, which were actually less durable than Imperial ships, in terms of wounds, but clearly were meant to survive any impact to get their payloads (marines) in play. While they were still good in a fight, they were clearly built to NOT be as good as real ships of the line. Their strengths were in boarding actions.
Eldar were very fun, and their strength was actually in some amazing escorts (smallest class of ship). Their battleships and cruisers were gorgeous, but they were meant to be an Eldar Raiding fleet, because the Craftworlds don't go to war. They used Solar Sails and could do a MSM just like their jetbikes, and had some amazing firepower, but they were utter glass cannons, if you could catch them. Later on they added a Craftworld Eldar fleet which had great cruisers and shitty escorts, but same basic principles.
Orks were... orky. Guns firing random amounts of firepower, ships dedicating to ramming... Honestly I'm not an ork person, I didn't pay much attention.
Tyranid ships were utterly customizable, in their role as living organisms. Another fleet VERY good at getting into boarding actions.
Necron ships were fucking amazing. The technology that Necrons had made them insane. They were the fastest ships in the game, with the best armor and defenses, and their weapons were disgustingly good, if shorter ranged. But their tech was precious, and if you lost half your fleet, you'd phase out, and took huge penalties for leaving a ship behind.
Tau were... different. In a game with massive broadsides, Tau were the ones that were designed specifically for the Greater Good. Every ship almost NEEDED other ships to support them. They were built for massed fire, and mostly front arcs. An entire fleet closing in and picking people off. FW also made their own Tau fleet, designed very much with the swept-wing style of Mantas and Barracudas. They also had the Demiurg ships, which is why every time Tau rumors pop up in 40k, people think the Squats/Demis will appear.
The last fleet they published was Adeptus Mechanicus Explorator fleets... Basically these were Imperial ships with all sorts of advanced refits.