Reading Beedo's excellent Black City Campaign logs, we were treated to his players manhandling a dragon, thanks to a serious bum-rush, a killer robot, and some goofy rules that come with dragons. And some commentary on dragons.
Seriously, dragons get the shaft in D&D, or at least in the older versions I used to play. Everyone knows their HP, damage, etc. They are often caught sound asleep. Unlike other monsters, they don't just get to decide on their attack but instead have to let the dice decide, and they can be subdued.
To be fair to Beedo, he's playing them by the book, and specifically by the Red Box book. I'm more familiar with the AD&D ones, so I'm going to chat about them here.
Dragons need some changes to make them scary.
The usual approach is more hit dice, more flying, more attack modes, more more more. But I think there are some simple changes you can make to how you run them, and one to how they fight.
Some changes you can make easily:
Decide For or Against Breathing, Don't Roll - Demons don't roll to see what spell-like power they use, mages don't roll to see what spell they cast ("My enemies are bunched up and I've a Web spell ready! Okay, roll, roll, roll, I'll cast Audible Glamour.") So why should dragons? Even the stupidest ones are smarter than animals. If they have a tactical choice between claw/claw/bite and a breath, do the one that makes the most sense for the dragon. If they have spells, let them diced how best to use them.
And yeah, some of them are stupid and impulsive and bad tempered. Play that. If the smart move is to claw/claw/bite the fighter in front of you or breath on the group of archers, but your red dragon just got painfully nailed by a wand of cold from that wizard . . . breathing on the wizard makes sense. It would be in character, and take away some of the calculation the PCs can make about its actions. It's still an elemental force of terror, and a manifestation of some of humanity's worse flaws (sloth, gluttony, greed, pride, bad temper, murderous rages, etc.) so it might not react tactically if its character flaws are prodded.
Lower the Sleeping Percentages - Even Tiamat has a 10% chance of being caught sound asleep. Any other monsters that vulnerable? No. But sleeping dragons are part of the myth and canon, so use it . . . but drop them. Cut all percentages down by a factor of ten. White dragons at 60% chance of sleeping? Make it 6%. Red Dragon? 2% Tiamat? 1%. If it's sleeping, check early and often to see if it wakes up.
No Subdual Rolls - you want to subdue the dragon? Good, whip it down to 0 HP. None of this "roll to see if it is subdued yet" crap. Earn it. Heck, make them get it down to 0 and then have it roll to see if it's subdued - make it a saving throw if you want.
Finally, a rules change if you want one:
Random Dragon Breath - Okay, so let's say you think dragon breath is too weak. Make that damage the average. Dragon with 11 hit dice and 88 HP does 88 points of damage. So make that 22d8 damage - that averages to 99 points, with a min of 22 and a max of 176. Scary, and it gets rid of that auto-calc. Basically, double its HD and roll that many dice. It'll average to the same but get some really swing-y nastiness sometimes. Especially if you like to roll 22 x 1d8 instead of d8 22 times.
Anyway, that's how I'd do AD&D dragons if I was running AD&D again. And by extension, how I'd modify dragons in a clone or in B/X or whatever, if I ran one of them.