Well, I imagine their breath weapon would be something similar to that of Bombardier beetles - because that'd be awesome. They would use this to scald and harm their enemies, and then attack.
A dragon would be large, but not as large as those in myth - myths tend to exaggerate size, after all. I'm imagining them as something roughly the size of a horse, with a large wing span that could fold up. They would be quadrepeds, with the "wings" being flaps underneath the forearms, similar to that of a bat's. These wings would not allow flight, but perhaps gliding - leading to dragons living in mountainous and hilly-cliff areas, especially areas with thermals that could raise the dragons hundreds of feet in the air. Because of their gliding ability, they'd be excellent climbers.
They would either be cold-blooded, living in warm climates (and often sunning themselves on rocks), or they would be warm-blooded reptiles (some say dinosaurs were not cold-blooded; dragons could be a surviving species). Even if they were warm-blooded, they would still probably prefer warmer climates to facilitate their flight (more thermal rises). warm-blooded actually makes more sense, as dragons are depicted as intelligent, and cold-blooded creatures are usually rather stupid.
They would be hunting carnivores, naturally - more similar to the komodo dragon than, say, crocodiles.
Their bite would have a necrotic toxin that would cause cell deterioration (similar to a komodo dragon). A large enough bite would eventually lead to weakness and death - dragons would use this bite to injure large prey, and then spend days tracking and harrassing their targets before going in for the kill.
Because they are gliders and are rather large, I see dragons as being nocturnal creatures - provided they are warm-blooded creatures. If they are cold-blooded, this could be a difficulty (more evidence they are warm-blooded!). Because they are night-flyers, they should have great nightvision or some other sense of detection. I don't imagine them having owl-sized eyes, or even really needing it (as they have a very flexible neck allowing for a wide range of vision). They could be very far-sighted, and lack close-range vision (relying on scent when they're close). I imagine dragons would be rather lacking when it comes to hearing - they may even be deaf.
Dragons would be intelligent, but far from human-intelligence. I imagine something close to a dolphin or other higher-functioning mammals. I can see them having a "hoarding instinct" for shiny objects which they use in mating rituals (decorate your lair, whoever has the nicest lair attracts a mate). Dragons are solitary, territorial creatures, and will fiercely defend their territory from intruders. Because they are solitary, they wouldn't have much "culture", which could restrict their capacity for intelligent behaviour.
Alternately, we could say that dragons travel in relatively small "flights", using group tactics to hunt. They would breed every few years on a remote island, in a manner similar to birds. This would allow them to have a "culture" of sorts - different flights could have different vocalization patterns and hunting behaviour.
