So iconic as to be part of the title of the game, dragons have undergone quite a transformation over the years. They started off as being a threat, one might even say a grave threat, but a threat that could be pursued without shitting yourself in the planning. Sure, they were dangerous and deadly, but they could be beaten.
For purposes of comparison I'll be looking at 100 year old red dragons from Monsters & Treasure and the SRD3.5. That age is Old for an OD&D dragon, Young Adult for the 3.5 variety.
OD&D dragons all had AC2. Since there is no mention of multiple attacks or damage bonuses, it may be inferred that all dragons had one attack for d6 damage, apart from their breath attack. Other than that, they were each different. Reds had 9-11 HD, with Old Reds getting 5 points per hit die. That was also the amount of damage done by their breath weapon. They were able to breath 3 times per day. They hit AC2 on a 7 or better.
Their OD&D adversary, a Fighting-Man of similar 10th level, would likely have fewer hit points. With the average roll of a d6 being 3.5, he would have 36 points, compared to the dragon's 45-55. Of course, if the F-M had a +1 CON bonus his average HP jump up to 46, with a max of 71. His Saving Throw vs the dragon's breath is 8, which means he will save 65% of the time, for half damage. The bitch of it is, even if he saves, the dragon's 3 breath attacks per day are more than enough to roast our F-M without some sort of magical aid. That's ok, though, because he only needs a 10 to hit the dragon. Hopefully by the time a 10th level fighter tangles with a dragon he will have a pretty potent magic sword, some armor, maybe a Potion of Heroism. In other words, he should have a chance, even if he wandered up on the beast accidentally.
In comparison, the 3.5 dragon is almost a demigod. I'm not going to completely break down a Fighter's chances against this beast because I'm not really versed in 3.5-speak. I'll compare things that are obvious, but nothing that gets "rulesy". A 3.5 red will sport 218 HP. The Fighter (we'll say he's 19th level, to put him on par with the dragon) would have, on average, 105 HP. Of course, this is 3.x, so it is safe to assume a CON bonus, but even if it is a massive +5, his hit point will only (only?) be 200, on average. He could possibly have a max of 285, but that's not very likely.
Our 3.5 red is a Young Adult at 100 years. That translates into 10d10 damage for the breath weapon, which recycles (1 in 4 chance per turn of being available). That's a potential of an average of 55 points every fourth turn. Of course, you could go the whole fight with the poor red not hitting that one in four, or he could nail it every turn, in which case the Fighter is screwed. But, maybe he's screwed anyway. See, that Young Adult is like a B-17 bomber; it has weapons hanging all over it. It gets the following attacks: 1@2d6, 3@d8, and 2@d6. They all have a +19 attack bonus. A Fighter in half plate (which seems closest to D&D plate mail) is AC 18 without DEX or magic. He better have plenty of both or that dragon is going to rape him for an average of 28 points per round, not including breath.
I want to be clear about something. I'm not trying to compare OD&D to 3.x. I'm using 3.x in this comparison because it is the final culmination of a game with the D&D monicker that actually resembled D&D. It is the ultimate end of the dragon's development cycle in a recognizable form. I'm not even trying to say one is superior to the other. As an aside, I believe that what happened with the dragon is a direct reflection of the design paradigm of later editions. That being character power inflation and monsters becoming more and more challenging occurring an endless and self-serving cycle.
So, you can see that at some point the dragon went from being a worthy adversary to being a virtual deity. I think it also illustrates later editions' reliance on character buffs, special abilities, and magic items in order to overcome foes. A smart player running his character with a smart referee can handle an OD&D dragon without needing to use a database to cross-reference all the necessary bonuses. I don't believe any amount of player skill will help a later edition character who is short of his bonuses when he runs up on a dragon.