Elvish realms of the 1st and 2nd age vs 3rd age
 

I've always wondered about the difference between the Elvish realms in the 1st and 2nd age where the first born were still mighty and lofty, compared to the 3rd age where only a few elves were left but with elven rings of power. Doriath seems to be the fairest elf-kingdom in the first age as well as Gondolin. But because their rulers did not have the rejuvenating enchantments of the Three, do you think these kingdoms were outmatched in beauty and homeostasis by Rivendell and Lothlorien? After all, the Three can ward off decay and weariness and sustain enclaves as beautiful as the true West. Melian was a maia but I do not think she had any power to rejuvenate the realm she fenced and the people about it. Realms like Nargothrond, Doriath, Gondolin, and Englarest did not have that perfect "homeliness" that Rivendell have, or that dream-like, ethereal feeling evoked in the forest of the Golden Wood.

Well, if you ask me, I'd think that for example when Melian, as you say, protected Doriath, there was nothing that would need to rejuvenate, because nothing could be harmed.

But most importantly, I think actually in the 1st age the Rings were already not needed. The Firstborn were strong and the world didn't yet start to fade, so there was no need to rejuvenate anything. Remember Treebeard's words and his song about the forests he visited, for example. All the big losses came with the end of the 1st Age, when all of the wonderful lands of Beleriand was lost. And then, the Rings were able to bring back some memory of Doriath, Gondolin... in Laurelindórenan and Rivendell.

I disagree with Legate on the matter of the need for rejuvenation. Ever after Melkor marred the spring of Arda, all things started to decay (or more so rapidly). In the Atrabeth, even Finrod admits to Andreh that the high-elves who returned from Valinor already feel the negative effects of Melkor's marring of Arda upon themselves.

Well, on this I can say only two words: not canonical :p

But seriously now, of course you have a point. But still, you have to agree that there is big difference between the kingdoms ruled by Elves, full of Elves, cared of by Elves who came from Valinor and the memory of the Light was still in them, and the fading, barren, empty lands without Elves (who parted to the West, mostly) in the latter Ages (2nd, 3rd).

Well, the Finrod bit may lie with a less canonical text, but the marring of Arda's spring is referred in the Silmarillion too. My general take on "canonicity" is that if an information is not refuted somewhere else, implicitly or explicitly, then it remains valid.
Indeed. Even the marring I mentioned has a progressive effect in time, and the last paragraph of the Silmarillion reflects your idea also - the passing "from the high and beautiful to darkness and ruin".

The doom of Mandos was already set in motion, and everything east under the sun must dwindle and decay. So why would you say Doriath couldn't be harmed? Well of course it couldn't be harmed by its adversaries but it never states that it was an ageless land without stain -- just that evil things did not come there while the girlde lasted. Maybe during the starlit years but under the moon and sun, the east was to decay.

Hmm, yes, you got a point. However, we are back at the fact that Melian was there, and she was Maia, which could means powers to stop the decay, or not? I recall all the images of flowers blossoming under Lúthien's feet and so on... I am not expert on this period, however, so these are just thoughts, unsupported by any direct evidence...