I actually think something like 75% of the criticism comes in the fact that you are rendering a Star Trek ship. That looks a lot like it could be a still from a battle scene in DS9, although there are some minor inconsistancies as have been pointed out already.
Don't put in hulks, because Star Trek has never suffered from the Battle of Endor syndrome, in part because the ships were more powerful, smaller, and rarer. Another factor is their vast speeds; even at impulse, a battle would leave wrecks very, very quickly. The massive fleets of ships have their places in things like Star Wars, but in Star Trek it seems forced.
The lighting needs to be adjusted, so that the Defiant is backlit in a greenish hue, and all of the glowing bits are illuminating the hull around them. The phaser pulses also need to light up the front of the ship a little more. It would be nice too if you either found a render-worthy ship out of some other universe (honestly, Star Trek renders are a dime a dozen... this is actually one of the better ones, but it's still a Star Trek render) or created your own ship and showed it off this way.