Resolution is easily one of the first things a pixel artist needs to think about before they even start. It influences the art style, and as already mentioned it influences your working speed.
Ahriman's prophecy isn't as great as you think it is. Their door is taken straight from default RTP assets that comes with RM2k. The tree looks a lot like the tree used in FFVI:
http://www.spriters-resource.com/snes/ff6/sheet/6653/. Lots of stuff were taken from that First Seed Material site.
As a result from taking from multiple sources, their art style is inconsistent. RM2k RTP and FSM assets are a lot more cartoon than the more realistic looking FFVI trees. Dunno how obvious that mistake is to others but inconsistent styles would make things look more fake than it needs to.
As for the pixel art itself, a couple of stuff I might be able to bring up. The rendering uses a lot of single pixel dots which generally looks much less polished than pixel art that is done focusing on pixel clusters (blobs of colors created from same colors touching each other). That is what makes some of the best pixel art look really polished. A low-res game I think has a solid art style with solid rendering is LoZ: The Minish Cap, if you wanna see a better example of low resolution pixel art that looks polished.
To add to what PixelPileDriver said, you're gonna want to know what the game screen looks like. It doesn't matter what a game asset looks like by itself. It's how everything meshes together into a game screen that matters. For the heck of it I just scribbles down something very basic, but you may want to start with blobs of colors just to make sure the color scheming for the environment is solid.
This is 512x384. Seiken Densestu 3 is a game that uses a similar resolution but 25% the size you're working with - 16x32 characters (more or less), 16x16 px tiles, 16x14 tiles on the screen. SD3 doesn't have any glaring problems compositionally, so I'm just gonna use them as a shortcut to say that 512x384 may be a game resolution to consider. Trees will likely be a nuisance blocking the view for being the size it is but it's probably better than smaller bonsai trees that are obviously copy-pasted.
1400x800 though, is too big, even if you didn't mean to ponder over that as a screen size it's still a convenient number for what I need.. This setup is probably good for a strategy game which individual units are merely your pawns, but it's innappropriate for a typical RPG when the game play puts emphasis on the hero. Also look at how the trees are now proportionally smaller, so you need more trees to make this scene look like a forest, and it's suddenly more obvious that this tree was blatantly copy-pasted. The more tiles on screen, the uglier the tiles look. You're also gonna get lost getting distracted by a gazillion different kinds of distractions when your hero is puny and seemingly insignificant.
Whatever sizes you work with, if they don't look good in your intended game resolution, they probably won't look good either if you try sticking them into a mockup as mere scribbles like what I did.
Also, a fun fact: you won't need anything faster than 60fps because that is about the rate human eyes work.