I have a few observations for you. They're not positive, but they're not meant in a mean way - they're given objectively as someone who's seen a lot of decks and who does consulting work for deck designers. It's nothing against you personally and they're given in the spirit that you should take them, learn from them and make something BETTER.
As a theme, elements have been done quite a bit - they're not always successful, either. The theme tends to not be as appealing - there's no characters, no storyline, it's just the elements. Visually, they're not as interesting as even some of the heavily-overused themes such as steampunk, zombies and Lovecraft Mythos. You're trying to make up for that in the court cards, making the elements into characters after a fashion, but it feels a little forced and by sticking so closely to the narrow color palette for each element in your court cards, the end result is visually not as eye-catching as standard, multi-colored court cards. They don't really stand out against your spot cards in any way that makes them appear special.
The diagonal line you're using to divide your cards seems like a primitive solution to the problem of making a two-way card face. It stands out all the more because you're using it not just on the court cards, but on every card. Why do this at all? Either make a proper two-way design without a border line separating the halves/faces, or toss two-way out the window - lots of people make ONE-WAY faces these days as well, and they can be quite beautiful when done well. You can especially do this with the spot cards (Ace through 10), make them one-way - in their present form, they all have twice the number of pips as normal, something that some people won't like.
The dealer button card seems a bit odd to include. It's also unlikely to be in the final product, depending on whom you've chosen as your printer - the US Playing Card Company (USPC) uses a 56-card deck sheet, but these days they're more the exception than the rule; more card manufacturers are using a 54-card sheet and will charge you extra to have more than 54 cards in your deck. That means your dealer button card either won't be printed or will come at the expense of one of your jokers. It makes more sense anyway for a dealer button to be just that, a button: a largish, probably round, three-dimensional object that won't accidentally get confused for being one of the cards in the deck.
The art is a little cartoonish. Some people find this appealing, some don't - test the waters first, find out if there's enough demand for art like that on your deck. Regardless, I suggest you eliminate the cartoonish indices. Indices are meant to be simple, clean and easily read with a high degree of uniformity from card to card. Cartoon artwork doesn't fit the bill and reduces the functionality of the deck.
Your design uses a four-color pip design, sometimes referred to as a "no-revoke" deck because it makes it easier to avoid mistaken plays in games like bridge due to confusion over suit and color, confusing a red heart for a red diamond or a black spade for a black club, etc. You should know that four-color decks are not as popular outside of bridge, and most bridge players don't use poker-sized decks for playing because of the number of cards they have to hold in their hands. Many solitaire games actually become difficult to play using four-color decks because they were designed for two-color decks. In general, straight-up four-color decks such as yours aren't as popular, at least not without some serious design tweaks.
Regarding the card back, the design suffers the same flaw as the faces in regard to having a large border dividing the two halves of the design. Also, it's practically the kiss of death to place the deck's name so prominently on the back of the card. Visually, it's as subtle as trying to drive a nail into a wall for hanging a picture by using a two-handed sledgehammer. The imagery alone should be enough to let people know what deck this is, as well as the standard titling on the Ace of Spades and the tuck box. Most decks I've seen - decks that aren't advertising a product or aren't licensed merchandise for a product of some kind - have no writing on the backs at all, and the few that do have it in a subtle manner and often in Latin, to make it seem more integrated with the design rather than standing out like a billboard.
Whew...that was a lot...
Now, consider this - not every word I just wrote is made of spun gold, nor was it transmitted to me from the heavens on high. I'm human, as flawed as the rest, and can just as easily be wrong about the things I've stated and suggested. You might strongly disagree with a lot of what I said - good. If there are certain things about your design that you're resolute about, you should as the artist stick to your guns. But hopefully, you're able to see that there is at least a grain of truth in most if not all of what I said, and if you disagree with some of it, that you at least have solid reasons for disagreeing other than the fact that you don't like my opinion. I often tell designers that there are certain rules to design, and that yes, you can occasionally break a design rule - but you need to have a very good reason for doing so, not simply because you didn't know any better, or the end result won't be well received. If you want to do something in a specific way, have a very good reason for it, especially if it's a way that flies in the face of convention.
Welcome to the world of playing card design and best of luck on your project.