Spoiler: why do I think Druid's spellcasting is great
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Because while lacking the best spells of several categories, they do still get an impressive array of spells, from the most generalist to the most "utility niche", including...
- Some of the most useful environment control spells (Plant Growth, Fog Cloud, Moonbeam, Dust Devil, Wind Wall, Wall of Water, Sleet Storm etc up to EarthQuake and the like)...
- Some of the most useful buff spells (Enhance Ability, Darkvision, Longstrider, Flame Arrows, Freedom of Movement, Polymorph, plus self like Warding Wind for gishes but that's another topic)
- Many useful healing/purification spells (Healing Words, the now op Healing Spirit, Lesser Restoration, Greater Restoration, Heal, Heroes's Feast, Primordial Ward).
- A great variety of exploration/investigation utility spells (Locate spells, Identify, Goodberry, Purify, Dispel Magic, Find the Path, Scrying),
- Some spells, some of them rituals, that can completely change entry or fighting tactics or warring strategy provided the situation fits (Pass Without Trace, Water Walking / Water Breathing, Transport by Plants, Wind Walk, Conjure -Air- Elementals)
- A decent enough array of AOE, which have generally much weaker damage than direct counterparts from Sorcerer/Wizards but compensate with equal or greater range and added effects (Flaming Sphere -great for a time-, Earth Tremor -small radius so friendly, puts prone, cheap-, Erupting Earth (damage and difficult terrain), Call Lightning (good damage per concentration, large range), Tidal Wave (particular shape that seems to be adaptable), Sunbeam (great against darkness), Ice Storm (unusual shape good for low flyers and avoiding friendlies), Sleet Storm (great range, obscures area, makes people fall prone, difficult terrain) etc...
- A pretty good array of AOE or single-target (with some upcastable) debuffs: Thorns Whip (yeah, being moved can be a pretty strong, although incidental, debuff ;)) Heat Metal, Hold Person, Polymorph, Dominate, Grasping Vine (same as Thorns Whip, party dependent so meh or great YMMV), Elemental Bane (once per turn, but every turn, plus lose resistance... ;)), Sympathy, Wall of Thorns (like Plant Growth and Spike Growth fused together)...
- And the best creature conjuration spells until very high level (in PHB, IIRC there have been several conjuration spells added in other books but I don't know them).
AND... You can design a different set-up among ALL the spells, EVERY. SINGLE. DAY.
You wanted to try a spell but it has been pretty underwhelming in your experience... You're a Bard/Sorcerer/Warlock? Tough luck, you have to copie with it until next level (which can end as pretty long past level 8-9). You're a Wizard? Well, you can prepare another spell another day, but that's a few pages of your book you'll probably never read again. You can just hope that your DM will throw a few bones (read: spells as loot/trade) at you soon.
You find that particular spell potentially great, but you are afraid you'll never see a chance to really use it (like Water Breathing)? For every class except Cleric and Druid, you have a tough choice to make because it's taking one precious "spell known" slot for a while, confer above.
Druid is the only one that can really use all the spells, even the most niche, because you don't care about reducing your "always useful" spell list by one-two "slots" if it's only for a day.
And if you really like those spells, Land Druid brings not only extra slots, but also some potentially "external" spells from other lists that are very useful (Haste, Slow, Cone of Cold, Silence, Web, Dream, Stinking Cloud, Greater Invisibility)