There are ten 6th-level druid spells in AD&D, up from eight in OD&D. Seven of those spells have been brought forward from OD&D. Cure Critical Wounds has been brought across from the cleric spell list. Fire seeds and wall of thorns are both new spells. The OD&D spell list had finger of death on it, but in AD&D that's been move up to 7th level for druids.
Animal Summoning III: Works like animal summoning I, but calls up to 4 animals of 16 Hit Dice, 8 animals of 8 Hit Dice, or 16 of 4 Hit Dice.
The OD&D spell is said to be a "quadruple strength" version of animal summoning I, but as with animal summoning II there's no indication of what exactly that means. Hit Dice? Numbers? It's all up to the DM, but I would be inclined to apply it to numbers, as the OD&D spell summoned animals based on size rather than Hit Dice.
Anti-Animal Shell: Creates a globe around the caster that prevents the entrance of all animal life of a non-magical nature. This is a case where the word "animal" is used differently than usual for AD&D, because here it basically means any living thing; giants are specifically called out as being affected. Undead, aerial servants, demons, and devils are said to be immune, so presumably that rules out any other extraplanar beings. There's a large grey area here for things like dragons and manticores and the like; this is a spell that could cause some real debate at the table as to what monsters count as magical.
The OD&D spell hedged out "basic animal types", such as giant animals, birds, insects, and reptiles, but excluding fantastic animals (centaurs, gorgons, etc.). There's a bit less ambiguity there, I feel, but the spell is also not as powerful. It also precluded those inside the shell from attacking creatures outside, which the AD&D spell doesn't do.
Conjure Fire Elemental: Opens a plane to the Elemental Plane of Fire and summons forth one of the following (determined randomly): a 16 Hit Die Fire Elemental; 2-4 Salamanders, an Efreeti; or a huge Fire Elemental of 21-24 HD. Because druids are connected to nature and elemental forces, they don't have to worry about the elemental attacking them, and they don't need a protective circle. The elemental pretty much does what the caster wants, and stays until the spell duration ends, or it is killed or dispelled. The reverse of this spell - dismiss fire elemental - can send it back, as can a dispel magic, but salamanders, efreet, and the strongest Fire Elementals can only be dispelled by druids.
The OD&D spell was much the same, but the druid could only summon 1-3 Salamanders, and had no chance of summoning an Efreeti. It also had no reversed version.
Cure Critical Wounds: Same as the cleric spell, except that it needs mistletoe.
Feeblemind: This spell can only be used on a spellcaster, and it reduces the victim's brain to "that of a moronic child". This can be cured with a heal, restoration or wish spell. Various different classes get modifiers to their saving throw, with clerics the least susceptible (with a +1 bonus) and illusionists the most (with a -5 penalty). Non-humans get a -2 penalty, which I'm not sure how to apply; is it cumulative with the class penalty, or is it instead of the class penalty? Anyway, the effect of being feeble-minded isn't spelled out at all. Obviously it stops magic-users and illusionists from casting spells, but what about clerics and druids? Their spells are based on wisdom, so the ability should still be there. Although now that I think of it, feeblemind doesn't necessarily just target Intelligence; it targets "the brain". So yeah, I'd rule out spellcasting for all classes, as well as using wands and scrolls and such.
The OD&D spell worked in much the same way, but it could only target magic-users.
Fire Seeds: Creates four acorn fire seeds (that can be hurled as missiles) or eight holly berry fire seeds (that can be set to detonate with a command word). The acorn fire seeds must be thrown, and deal 2d8 damage to anyone within 1" of the explosion. The holly berries are set on the ground, detonated with a command word, and deal 1d8 points of damage to anything within 1/2". It's nice for druids to get some sort of damage spell (especially one that's not dependent on prevailing weather conditions), but it seems a little weak for a 6th-level spell to me. Perhaps I'm just unfairly comparing it to the magic-user spell list; druids aren't supposed to be offensive casters, really.
Transport Via Plants: Allows the caster to enter a plant and exit from a different plant of the same species anywhere else on the planet. (Actually, it says "regardless of distance" - could it be used to travel to a tree in another dimension, or on another planet?) There's a small chance (which gets smaller the higher-level the druid is) that the caster will be sent to a plant 1 to 100 miles from the intended location.
The OD&D version of the spell required the destination plant to be one that the druid has seen or heard about (that's not the case in AD&D). It was also limited in that the caster could only use it once per day; the AD&D spell can presumably be cast as many times as a druid wants to memorise it.
Turn Wood: This spell pushes back or splinters any wooden objects in the spell area, although it doesn't affect anything over 3 inches in diameter that is firmly anchored. That's a shame; I had images of a druid knocking a village over, but I figure that houses would count as being anchored.
The OD&D version of the spell is basically the same.
Wall of Thorns: Creates a wall of thorny brush that deals 8 points of damage to anyone that tries to break through (or is otherwise pushed into it). This damage is modified by the victim's Armor Class, so the more defenses you have the less damage you take. (This is a really good mechanic, it's a shame that D&D doesn't make more use of it.) The thorns can't be burned with normal fire, but magical fire burns them away in two rounds; however, for those two rounds it functions like a wall of fire spell, which is pretty rad.
Weather Summoning: This is another powerful yet vague spell: it can do a lot, but it's impossible to give mechanical effects for what it does. In general, the caster can change the weather pattern based on what the current season is: tornados or thunderstorms in spring, torrential rain or a heat wave in summer, fog or sleet in autumn, cold or blizzards in winter, etc. This is dependent on climate as well - obviously things in the Arctic would be different, and I feel like Gary is applying his own personal experiences to the examples given. Still, it might be a reasonable guide to the seasonal conditions in and around the Greyhawk area. The weather takes 6 to 17 turns to arrive after the spell is cast, but it is in no way under the caster's control. Several druids can work together to create even more powerful effects. The power of the spell is dependent on the druid possessing greater mistletoe (mentioned a few posts back), and the effects will be weaker without it.
The OD&D spell works similarly, with a few numbers tweaked here and there.