Giants: There are six types of giants detailed in the Monster Manual: Cloud Giants, Fire Giants, Frost Giants, Hill Giants, Stone Giants and Storm Giants. The first five debuted in OD&D, while Storm Giants were first seen in Supplement I.
The first thing that leaps to my attention is that giants are said to have Strength scores ranging from 19 to 25. This is the first time we have seen mention of ability scores greater than 18. I don’t think these scores get detailed until the Player’s Handbook, but they're factored into the giants' stats in a vague sort of fashion.
There’s a method given here for determining the stats of young giants. You simply roll percentile dice, and the percentage rolled is how close to fully grown the giant is. Their hit points and damage are adjusted by the percentage rolled. This seems like a good enough solution to work for just about any monster, and I’m going to use it for monsters where this stuff is not already detailed (i.e. dragons have the whole age category thing going on, so I wouldn’t do it for them).
Giants have gotten poorer on average, it seems. In OD&D, a giant always carried with it a sack containing 1000 to 6000 gold pieces. Now they carry 1000 to 6000 coins, which could be anything from copper to silver to gold to platinum.
Cloud Giant: Cloud Giants are now encountered in smaller groups than they were in OD&D, have a better Armor Class, move faster, and have just a few more hit points on average. In OD&D they were either Neutral or Chaotic, but now they are Neutral with either good or evil tendencies.
Giants were pretty sparsely detailed in OD&D, so we get a lot of new info here. Firstly, we learn that some Cloud Giants actually live on clouds, which cements them as being inspired by Jack and the Beanstalk. In OD&D all giants had a chance to have pets, those being hydras, bears and wolves. These pets have now been individualised for each type of giant, and Cloud Giants get spotted lions. A specific range and damage are provided for the giant’s stone throwing ability, and they can now catch any large stones hurled at them (this applies to all the other giants as well). While previously Cloud Giants had been said to have a keen sense of smell, that has now been translated into a mechanical bonus, as they are very hard to surprise. And logically, given that 10% of Cloud Giants live in the sky, they can now levitate themselves. We also get a physical description for Cloud Giants; they have blue skin and silver hair.
Fire Giant: Fire Giants have a slightly better Armor Class, but are otherwise statistically similar. Neutral or Chaotic in OD&D, they are now Lawful Evil. They are still immune to fire (and this includes red dragon breath, so I’d extend it to pretty much any fire there is). Their pets of choice are hell hounds. They have black skin and red beards.
Frost Giant: Frost Giants have a very small amount of extra hit points, and their alignment has changed from Neutral or Chaotic to Chaotic Evil. They are also a little shorter on average (15’ instead of 18’). They keep winter wolves as pets. They’re immune to all cold attacks, including dragon breath, just like they were in OD&D. They have dead white skin and blue or yellow beards.
Hill Giant: Hill Giants now appear in greater numbers, and get a couple of extra hit points. They’re also a little shorter on average than they were (10½ feet instead of 12). As guards they keep dire wolves, giant lizards, or ogres. These guys are coloured pretty much like humans, and that’s basically what they are – very big dumb humans with clubs.
Stone Giant: Stone Giants have gotten a major boost to Armor Class – from 4 to 0. It looks like the ‘Stone’ part is starting to apply literally. They could be Neutral or Chaotic in OD&D, but now their alignment is listed as Neutral. Like most of the other giants, they’ve shrunk on average, from 15 feet tom 12 feet tall. They keep cave bears as pets. In OD&D Stone Giants used the heavy catapult rules for stone throwing, as opposed to the regular catapult that all the other giants used. Here they’ve retained their rock-throwing proficiency, getting a better range and damage than everyone else. They’re also better at catching rocks than the others as well. And I like how they are described as playful, like they throw rocks at people just for kicks (it reminds me of the Stone Giant scene from The Hobbit, actually). They have gray-brown skin that helps them blend into rocky surroundings, but aren’t given a surprise bonus or anything for it.
Storm Giant: Storm Giants are now encountered in smaller groups, but they have a better Armor Class and a few more hit points. Their alignment has changed from Lawful to Chaotic Good. They’re shorter on average than they were, going from 24 feet to 21 feet. They keep rocs and griffons as pets, and may even use a roc as a mount. Some storm giants live underwater, and they may have sea lions. Surprisingly, storm giants couldn’t cast lightning bolts in OD&D – but here they can summon an 8d6 bolt once per day. They also get some other new abilities: levitate, predict weather, call lightning, control winds and weather summoning (some of these replace the giant’s old Control Weather ability, which I guess is a spell that doesn’t have that name or doesn't exist in AD&D). They’re now immune to electrical attacks. Physically, they have green or purple skin, with green, blue or purple hair.