Worldbuilding can be one of the most fun parts of creating a fantasy story. I claim there are four prime traits that makes a good fantasy (or sci-fi) world.
One of the biggest challenges when creating a radically different world is to make sure it could actually exist. A prime example of failure is Harry Potter. Rowling introduced a magic system with no apparent cost to use or limits on what it can do, and a society of people who have it, but as far as we can tell, those people live similarly to muggles. Also, it's totally unrealistic that magic is kept a secret. With mages being so powerful, they could be millionaires if they were only to sell their services to muggles. If the argument is that magic makes your life so good you don't even care about having money, then first of all, that would have a tremendous psychological and spiritual effect on the people that live that way, second, what about doing magic for others out of altruism, third, what about telling your loved ones the truth simply for the sake for it, and fourth, money can buy more than just pleasure, it gives you power over the muggle world. With so many huge incentives to come clean with muggles, it is quite far-fetched that the ministry of magic has been able to keep it a secret.
Another example of failure in this area, for a different reason, is Mass Effect. These games show a world where several spacefaring races live together, but there are never any language barriers. I've heard it's actually stated in the codex that it's because the omni-tool translates everything in real time, but that explanation raises more of its own problems than it solves. For one thing, anyone who's studied a foreign language knows that machine translation has certain inherent limitations. In particular, with some real-life languages you often can't even begin to translate until you've heard the end of the sentence, and a lot of important nuance may be lost in translation because there is simply no way to express the same meaning (for example, how honorific name suffixes are usually dropped when translating from Japanese, but this can really mess up a conversation in some cases). Also, what about the sound waves that are actually coming from the alien's mouth? Are they not still reaching Shepard's ears? If so, you would be hearing the alien and the omni-tool speak over each other, which would likely made it hard to understand - especially if every other character of a different species in the room also had their omni-tool speaking simultaneously in a different language. If the answer is that the omni-tool doesn't speak, but simply displays what they're saying on a screen, then you'd think you wouldn't need to go the codex to find out that that's happening. Shepard would be constantly looking at it during any conversation with an alien.
A more generally applicable point under believability is to remember to think about the basic necessities of life. If the world is similar enough to ours then these are usually already solved, but in the case of a radically different world you have to answer questions like, How does your society produce food? What kind of houses do they live in? How do they get around? What do they do with their waste? If you fail to answer these questions, you're left with the worst possible result: a tower of Jenga blocks falling over because you neglected the base.
If you've created a cool world, it's all for nothing if you don't show it to the reader. Stories set in fantasy worlds should generally try to show off the world as much as you can. The challenge is how to do this in a way that doesn't distract from the plot.
Fantasy worlds are cool because they're different from our world. Our world is boring. Nothing cool happens in our world. (Or, to the extent that it does, we're influenced by "the grass is always greener on the other side" bias.) So all other things the same, the more different a fantasy world is the better. The only reason it doesn't seem that way is because extreme differences make it hard to maintain...
If you make your world too different, you might make it hard for the reader to understand it or the characters who live in it. For example, if the world is 4-dimensional or the people in it are centipedes, what would it even be like to live there? The reader can't appreciate it if they don't know what it's like.