I think part of the problem is that gamesas were so excited about Dragons and Dragon shouts, they introduced them ridiculously early in the game, and as such, have to scale them down so that a level 2 newb, with the help of a few guards from Whiterun, can pwn a dragon. I actually thought that was a bit on the lame side. If Dragons in the Elder Scrolls lore are actually supposed to be powerful and threatening, then you've got not business fighting one before at least like level 20 or 25 or so - when you are conceivably strong enough to actually be able to defeat one.
So, they gimped out the dragons. That is, the problem isn't that giants are too strong - the game needs some powerful enemies for you to fight against at higher levels, and it's not unreasonable to have basically passive powerful enemies in the game, roaming the wilds, from level 1 - so long as you can avoid doing battle with them when you are too low level. The problem is that many of the dragons are just too weak.
Not necessarily... The first dragon fight is a scripted encounter, and could have been done with you never knowing how weak they were compared to a giant. You may have had a good idea (a couple hits to kill you instead of the 1-hit giant kill), but you wouldn't have seen it. Technically, they could have been rare right after that.
I definitely agree that dragons are not nearly as dangerous as they should probably be, but they are also capable of doing things that no other creature does (attack towns). This just goes back to whether or not you want to increase their difficulty and risk turning every city into a ghost town because they are told to attack a random town for dramatic effect, or making everyone essential in a game where people cry about not being able to kill kids because it "breaks immersion".
And to be entirely honest, giants should be able to kill dragons, IMO. After all, they are 3 times the size of you and swing a club as big as a small tree. Also, the whole "you have to be 20-25 to kill one" isn't a very good argument either. My first character was level 10-15 with 70 sneak. That's well beyond "newb level" for that particular skill. Practice any and only an offensive skill long enough and it will be powerful enough for the higher levels. The only thing your level determines is your health/mana/stamina, which is almost negligible when you have good armor as well.