If someone is writing a story that involves a character who is "elven" in appearance, they can go through and describe them, but due to common use in so many areas, the author pretty much gets a free ride when it comes to educating the readers on what the characters physical appearances are. Specifically, if you ask a bunch of people how elven folk look different from humans, a good number are going to say something to the effect of "pointy ears". What if your character has unique features like that, but is not an elf, dwarf, or other "common" fantasy species? How often do you need to remind your readers that your character's species only has 4 fingers on each hand? It's not exactly a crucial detail, unless your story is about how they learn to play piano or something, but it's still something that sets them apart from others. Due to my experience with MMORPG's, virtual worlds, and good old table top D&D, I have some favorites I like to work with. "Neko" are definitely my #1 (human form, with cat ears and cat tail), but there are other variants of the human form that I have tinkered with. So how often do you insert bits and pieces to remind someone that the character they are reading about is not 100% generic human? Is it even worth it to *try* and write this sort of character for a wider audience?
If you are writing from the perspective of a normal human, then it will be a lot more natural for the narrator to mention the physical features he/she sees, especially if they are new/unusual to him. If you're writing from the perspective of a character who is of the strange-looking species, then it will feel really unnatural if you mention his looks too much. In the case of a species with cat ears and tail, you can definitely find lots of subtle ways to mention it. For example, if the cat-human hears something, he might flick his ear in the direction of the sound. That will automatically remind the reader that he doesn't have normal ears. Or your character might have a tendency to move his tail a certain way if he's feeling angry or happy. Basically, you should try to use the character's unique features to express something other than just his looks, unless, of course, it is an introductory scene or something similar.
I'd agree with U.G. Ridley, it would depend on the perspective. If it's weird for the narrator, then it will be pointed out. If it's normal for them (as in, it's their own race looking like that), then it would seem weird to make a big deal out of it. If the person is the only one among the character with these traits, then there might be insecurity or pride it it which could be used (are they always wearing a hat to hide those ears, or something to show them off?). I think the main deal is to mention it as subtle as possible, and not trying to force it in to your readers heads. In either way, if it is the MC or someone worth remembering, I think it's easier for them to remember the weird things. That would mean that you probably won't repeat it a lot because THEY HAVE CAT EARS (for example) that is not a thing you forget. Though, it still doesn't hurt to mention it subtly anyway, as you would mention someones leg there was a reason to do it (no examples ) but, as mentioned above, no overdoing it.
As someone who read a lot of fantasy, I feel that you are making the common mistake of underestimating your readers. You don't actually need to remind them about the physical attributes of your characters very often. You need one good description, normally when that character/race is introduced or whenever they interact with a new character, from then on, just write in the movements/limitations of the character, tail twitches, ear wiggles, head turns, in ability to pick certain things up as you would with any human character. Going back to the description of the character to often can weigh down a story and it can be very repetitive. Especially if you do a detailed description of the non-human elements.
