What are the main differences between the Druid class in 5th edition vs. the Druid class in 3.5?
First, make sure you’re familiar with all of the general differences between 3.5e and 5e, as all of that is going to apply to a druid as much as it applies to everyone else.
The 5e Druid doesn’t get an animal companion, at all. That is the exclusive ability of a Beastmaster Ranger. You could take the Ritual Caster feat and cast find familiar, but familiars are a pretty poor replacement for the animal companion. Druids can still conjure animals, and the animal friendship spell does what it says, but it’s not the same as an animal companion who accompanies your adventures and grows as you do.
Like all 5e spellcasters, the 5e Druid is much more limited at high levels; they get very few high-level spell slots, and do not get more for having a high Wisdom score the way 3.5e druids do. This is part of the broader changes of 5e relative to 3.5e, but is well worth mentioning. Take care also to note the limiting factor that the 5e Concentration mechanic provides: since you can only concentrate on one spell at a time, your spells have to be chosen carefully with an eye towards making sure you have other useful spells to cast while maintaining concentration on one of them (5e Concentration does not require an action like it does in 3.5e).
Also, Druids no longer have the ability to spontaneously swap spells for summon nature’s ally spells—they have to prepare summoning spells if they want to cast them. The changes to 5e preparation mechanics makes this much less painful, however.
The 5e Druid still has Wild Shape, and it’s fairly similar in terms of what it does, but the details vary significantly. In particular, you get it at 2nd level, but by default, you can only shape into fairly-weak creatures, and the ability does not scale very much as you level up. By 8th level, a base Druid is limited to CR-1 creatures, and you never get anything better than that, nor ever get any plant or elemental forms. As a result, by default, by high levels Druids are using Wild Shape mostly for scouting and stealth, not for combat. The forms do have the advantage of providing a separate hp buffer (since you use the animal’s hp but then when you “die” you just revert back to being your regular self), but the limitations on your combat actions are quite severe until 18th level (when you get Beast Spells, see below). See the Circle of the Moon below, however, for the Druids that do get better Wild Shape.
The other thing about 5e Wild Shape is that it recovers after a “short rest,” which didn’t exist in 3.5e. That means that instead of spending 8 hours resting to get your wild shape uses per day back, you get your Wild Shape uses back after just 1 hour. Until 20th level, though, the 5e Druid gets fewer uses of it.
This was a 3.5e feat, not a druid class feature, but it should still be mentioned since Natural Spell was nigh-mandatory for a druid’s 6th-level feat: 5e does not have such a feat. Instead, there is the Beast Spells feature at 18th level, which is obviously a long, long time after 6th.
Numerous features of the 3.5e druid are not available to all Druids. Instead, Druids must join a Circle, and those features are offered to only those Druids who join the appropriate Circle.
Circle of the Land
Druids who join the Circle of the Land gain abilities similar to the woodland stride, trackless step, resist nature’s lure, and wild empathy abilities of the 3.5e druid. They also gain extra spell slots and bonus spells that they don’t have to prepare.
Circle of the Moon
Moon Druids gain powerful improvements to Wild Shape, making it more of the combat powerhouse that it was in 3.5e. They are also the only Druids to be able to Wild Shape into elemental forms.
The Circle of the Moon also offers an ability called Thousand Forms, which is the same as the 3.5e druid’s a thousand faces ability.
The Circles of the Land and the Moon were the two options in the 5e Player’s Handbook, but other Circles have been printed since then. These have features different from what the 3.5e druid got.
Both 3.5e druids and 5e Druids get Timeless body, though the 5e version comes at a later level and makes the druid age slower, rather than eliminating penalties (which don’t really exist in 5e).