When looking at comparisons between 3.0 edition and 3.5, I've noticed something interesting. In some cases, for example here and here, I've seen it commented that the Druid was not playable in 3.0 and needed buffs to make it work in 3.5e. Given that the Druid is a tier one class in 3.5, I find this surprising. I'm sure that if I tried I could find an extensive list of changes that would explain this difference in power, but that's not what I'm here to ask today. Partially in the interest of house ruling, I'm here to ask: what tier would the 3.0 Druid be if it was used in a 3.5e game (and why)?
In a campaign that otherwise adopts the 3.5 revision, an original druid would likely still be Tier 1
The druid was a Tier 1 class before the 3.5 revision. The changes made by the 3.5 revision only made the druid into a stronger Tier 1 class.
That's not to say that the 3.5 revision didn't change the druid significantly. For example, the animal companion class feature went from a reminder that a druid can cast the spell animal friendship to a proper class feature. Also, the druid gained spontaneous casting of summon nature's ally spells. Further, the supernatural ability wild shape added plants in addition to other minor changes. And the prohibition on weapons was removed, leaving only prohibited armor and shields to cause a druid lose her supernatural abilities and casting while so equipped and for 1 day afterward. These are in addition to other changes to the class.
(That host of changes, by the way, is what makes original druids unplayable in a campaign that uses the 3.5 revision, as per this answer; that is, a DM must revise original druids largely by hand for his 3.5 campaign.)
Nonetheless, these changes didn't alter the fact that both original and revised druids see every druid level advance a druid's caster level and gain a new spell level of spells every other level starting at druid level 3, the most rapid spell advancement in the game. Combined with the ability to prepare nearly any spell available to druids generally, the rest of the class's class features are gravy: it's this spellcasting that makes the druid tier 1, providing versatility that's paralleled only by another Tier 1 class, the cleric. (Versatility, to be clear, is largely what a class's tier indicates.)
The only way that an original druid could be considered less versatile than tier 1 is through comparison with the revised druid. That is, were the campaign restricted only to the core rules yet permitted both original and revised druids, and one player picked to play an original and another a revised, I suspect revised druid's player to feel as if his druid were a higher tier because of the revision's changes. This would likely be most pronounced at low levels. For example, at low levels the right weapon can make a difference (an original elf druid, for example, can't employ a bow), at level 3 a revised druid's dog animal companion is tougher than the original druid's normal dog gained from the animal friendship spell, and a revised druid's spontaneously cast summon nature's ally spell may solve a pressing problem or two right now while the original looks on enviously.
However, by the time the two PCs are level 7 or so, a lot of these differences will fall away in favor of just the spells, and that's where the classes remain relatively equal: blasting away with spells like call lighting and flame strike, creating defenses with spells like death ward and freedom of movement, gathering information with spells like speak with plants and commune with nature, and controlling the battlefield with, like, a third of their spell list. The druid spell list is really solid.
There might be an issue if the campaign mandates that an original druid use only original spells and a revised druid use only material from the 3.5 revision. (A list of these can be found here). I really can't recommend against this strongly enough: many spells were changed by later texts from their earlier appearances before the 3.5 revision, and "forcing" an original druid to use earlier iterations of spells that were later revised means the original druid will have greater access to unbalanced material! (While at lower levels it's possible this could equalize the two PC druids somewhat, at higher levels this will be a DM nightmare; hunt down the original version of the spell shapechange to see what I mean.)
Note: The dnd-3e druid and the original version of the spell shapechange can be viewed on this site. Class tiers are discussed in answers to this question. Apocryphally, Andy Collins was playing a dwarf druid in a campaign while overseeing the 3.5 revision, and the many changes wrought to dwarves and druids—changes sometimes considered by fans to be unnecessary power-ups—are a direct result of his play experiences.