This is the long awaited post for my point of view on this question. I finally finished it inspired by the recent post of the deviantartist karkajou1993.
EDIT: Now there is an updated and corrected blog post of this entry! Check it out: ktboundary-smnt2000.blogspot.i…
EDIT 2: the blog version has been updated with some new infos after an interesting post by Matt Martyniuk (link: dinogoss.blogspot.it/2013/09/y…). Go to read it!
1 - The ancestor of Dinosauria is feathered. Now we have proof that feathers were present both in theropods (Sinosauropteryx, Epidexipteryx, Velociraptor, Beipiaosaurus, Dilong, Anchiornis, etc..) other than ornithischians (Tianyulong, Psittacosaurus). Even if we cant 'rule out that the two groups of animals have developed these structures independently, it is extremely likely that the common ancestor of both possessed 'em. As it is unlikely that ungulates and felines have developed fur each independently from a hairless common ancestor, at the same time we can say that if Rahonavis and Heterodontosaurus were feathered is because even their "ancestor" was. Further evidence in support of this are pterosaurs's pycnofibres. Pycnofibre is nothing else but a bizarre word for structures most likely homologous to the proto-feathers of the dinosaurs. As the ancestor of Dinosauria is the same one of Pterosauria and since both orders are feathered, this archaic animal and all his descendants MUST be feathered.
2 - The polar dinosaurs MUST be feathered dinosaurs, then the categories of animals to which belong MUST be also feathered. Numerous data tell us that all dinosaurs were warm blooded, from gigantic sauropods to tiny Scansoriopterygids. Since every endothermic tetrapod (excluding cetaceans) is covered by thermoregulatory structures in temperate or cold climates, this reasoning can be applied to dinosaurs as well. Then the members of Neovenatoridae, Troodontidae, Dromaeosauridae, Tyrannosauridae (aka Deinodontidae), Ceratopsidae, ornithischian dinosaurs considered as Hypsilophodontidae, Rhabdodontidae, Spinosauridae, etc. are feathered, at least in temperate climates. This leads us to conclude that the ancestor of each group must also be feathered, and also its ancestor and so on. This brings us back to point 1.
3 - At different temperatures, different thickness (but NEVER TOTAL EXCLUSION) of plumage. Derivation of point 2. We said that Hadrosauridae and Ceratopsidae have feathers. But we have the fossil fragments of skin belongin' to both groups (Edmontosaurus, Triceratops) that indicate the presence of scales. So is step 2 wrong? Not necessarily. Just think of today's elephants that live in hot climates (and they still have some hair) and their relatives, the mammoths provided by a thick woolly coat. Or you can even mention rhinoceros and woolly rhinos, warthogs and boars, etc.. From this we can infer that dinosaurs possessed coverings more or less dense depending on climate and latitude. Moreover, the presence of scales does not exclude definitively the presence of proto-feathers small and sparse like the hairs of the current pachyderms. Further confirmation of this point is the skin of the afore-mentioned Triceratops: it's scaly but it presents quill knobs similar to those of his distant relative Psittacosaurus.
4 - Different weights, Different plumages. Derivation of points 2 and 3. Another factor to consider is the weight. In today's warm-blooded animals, in the EQUATORIAL AREA, species generally weighing more than a ton or a ton and a half have a sparse covering of feathers / fur. Similarly, the dinosaurs of EQUATORIAL CLIMATES that weigh over that threshold should not be heavily coated by integumentary structures. Proof: the small dog-sized Tianyulong has a thick plumage, while Corythosaurus, heavy as a rhino, is "hairless".
5 - If you are a carnivore like T. Rex, you are feathered. Derivation of point 4. So far we have only mentioned large herbivores big as elephants, but what about theropods of comparable size? Are they also to be considered scaly? Here comes the issue of metabolism. Citing Tyrannosaurus just to make an example (but if you want, you can do the following comparison with any other giant carnivorous dinosaur), this saurischian, despite weights as an African elephant, consumes an amount of energy equal to that of a 1-ton bear. This is because, although there may be a several tons herbivore with a metabolism more or less comparable to those of elephants, a giant carnivore of the weight of T. Rex (or heavier) can exist only in marine environments. Since Tyrannosauridae (and other large theropods of similar tonnage) do not live in oceans, since it seems that no land predator covered in fur actually lose his fur around 1588-1749 kg (like the bear Arctotherium) and since a carnivorous dinosaur that weighs 6 tons generates an amount of heat similar to that of a 1-ton mammal predator, there is no reason that big theropods must loose plumage. EDIT (April 2012): we've got the final proof of giant feathered theropods! Yutyrannus hauli (Xu et. al., 2012) is a giant, maybe Baryonyx sized tyrannosauroid from the Lower Cretaceous of China and it shows the unequivocal proof that big tetanurans had protofeathers. It's not very surprising if you agreed with the many points of mine, but it sure is a pleasant surprise.
6 - Not the whole body is feathered in the same way. When it comes to feathered or furry animals, we generally thought to the tegumentary thickness of the Star Wars's Chewbacca. We must however remember that the length and distribution of these coatings vary not only from species to species, but also from different areas of the animal's body as well. The hairs of the muzzle of some bovids are very short, while in other parts like the chest or shoulders may be long and thick (like in the bison). To also cite birds, vultures have thin feathers on the neck against sunburn, while having a more dense plumage on the body. Among suids and some other related species, some have fur (wild boars, peccaries) while others can be considered almost hairless (pigs, babirusa). Such disparities are also present in many other animals: rodents, insectivores, primates, etc.. so, albeit the previous points are not incorrect (in fact, I think they are not), the feather / fur factor cannot be entirely understood and there may always be some exceptions. In the world of dinosaurs, an example of this exception are Ceratosaurus and Carnotaurus. But you must also remind that exceptions are not the rule.
This post has been written not to impress or manipulate the minds of others or, even worse, indoctrinate forcibly what I think about it, but it is a plain statement that justifies the presence of feathers on my reconstructions of dinosaurs. When I'll put drawings of feathered prosauropods and carcharodontosaurids or whatever in my gallery, you now know that I have an outlandish explanation of why I have portraited 'em in that manner. I'm not absolutely saying that I'm an expert, I'm a genius while you're jerks, I'm smarter or other nonsense stufflike that. I just need to be understood. Or, if you prefer, pitied.
So avoid stupid, negative and quarrelsome comments. I do not need 'em, but most important of all, I do not want them. You want to insult, mock or make me change my mind for having expressed my idea? No thanks. As I do not want to do that to you, I ask you not to do it to me. I know, it's selfish and stupid, but this is my request. Then, critics must abstain.
I'm done with this. Good day to you all.
PS: Sorry for the bad english