I too agree that consciousness is most likely a matter of degree. That is to say, the more processing power an organism has the more its potential for a higher level of consciousness. Animals that are highly sentient consistently have brains with large numbers of connections and high neuronal densities (octopods, dolphins, crows, elephants, etc).
Which is why I'd be really surprised if we ever found out plants were conscious. They have very few connections in their nervous system and even something like a large maple tree with it's vast array of solar collectors (leaves) only generates about 200 millivolts of energy for the entire organism. Whereas something like the human brain all by itself consumes 12 volts (12,000 millivolts) to say nothing of the power requirements for the rest of our metabolism necessary to keep our brains functioning.
If we ever do bump into an advanced alien civilization I think there are some predictions we can make. That they will have a predatory past (meat is the food source with the highest energy content), they will have highly articulate digits/antenna/arms/legs/tentacles/whatever (necessary for building and using tools), and will come from a high oxygen non-aquatic environment (access to fire is also necessary for tool building).