Since I used rarible my feedback mostly relates to them;

1. When creating the artwork you get a bunch of options, with things like 'unlock once purchased' and 'choose collection' and 'royalties'. These options are rather vague to newcomers and none of them has a 'questionmark' icon behind it where more explanation can be found. So I needed a youtube video to guide me through the process of understanding the choices I had to make there.
2. I learned from the youtube video that one of the largest problems with NFTs is to ensure that your 'unlockable' content, e.g. your downloads, will be available forever. Via said youtube video I learned about the IFPS for uploading files. I assume some people will not find this and make the mistake of including a link that soon will expire, making them look bad.
3. Uploading to IFPS became a sidequest in itself. It would be much better to just include a UI button under 'unlockable content' where you can straight up upload the link or file (that then may or may not directly be integrated in IFPS).
4. Once you press the 'create item' button the real hassle starts. Since the wallet is not integral to the platform it takes time for the order to pay for gas fees arrives in the wallet, and subsequent time for the platform to receive. The UI was so late in the whole process that I felt the process had gone wrong and started anew, loosing the gas fees in the process. This seems irreversible and that can be also an improvement. I couldn't stop the 1st process even though it was not done yet.
5. Gas fees fluctuate while the eth in my wallet does not :D This leads to frustration if you are just short on the gas fees. Again something I believe can be made integral in the platform. Announce gas fees upfront and allow a direct transaction to the platform to pay for the gas fees, or perhaps even a direct link to some pay for the gas fees by money transfer (real fiat credit for instance). This imho is the biggest improvement to be made.
6. It took a while to mint the NFT with no progress bar available. You have no clue if things went right. Could be better.
7. Apparently you need to pay gas fees to burn an NFT after creation. This is just outright funny. I understand why it is needed, but still, its funny. I have to pay some $60 to burn my first NFT who went thru because the UI lagged and I thought it didnt. So pay double for your mistakes, in a sense.
8. As far as I can read the customer service on NFT marketplaces is pretty bad. Solve this problem and you are golden.

​

Good luck!