Here is list of small things that could improve usability of the UI. I can fix them later if there is any interest. Feel free to suggest more

General

• Aria tags are missing. I would add them to global components like hamburger menu button at least
• Open Graph tags seem to be completely missing. This is pretty important for content focused site
• Spinner when switching pages is quite small and not centered. There is nothing interesting on the screen at that point anyway, just make it bigger and centered
• Contrast seems pretty bad. In the heading especially. But I am no expert here
• Footer could use some centering and padding to better separate it from content

Login/register page

• Labels are not associated with input fields, so clicking on labels does not focus input fields
• Forgot password button is disabled but does not have disabled cursor on hover
• Buttons seem to be clickable multiple times when in loading state (causing multiple submit), they should be disabled
• HTML5 autofill attributes are missing
• Login button changes width when in loading state, this is little bit ugly
• Forgot password button does not have type="button, this could cause issues with triggering submit even of the form. However currently the button is disabled, so this does not manifest

Landing page

• Upvote/downvote buttons have tooltips of arrow-up and arrow-down
• I would add meaninful tooltips to upvote button/downvote button/score label/upvotes and downvotes labels
• Upvote/downvote labels could trigger upvote/downvote on click as well
• Next/prev buttons are quite small and insignificant on mobile

Comments page

• Empty comments page should show some zero-data element saying something like "Nobody has commented anything yet, be the first!", just to signal to the user that everything is loaded correctly, but there is no content.
• Empty comments page should not show comment filters
• Collapse thread button should
  • Be a button element, not span
  • Have a tooltip
• Tooltip on posted at label ("8 days ago") should have better formatting than default UTC one, something like "2019-1-2 18:23:35" would be much more readable than current one (which includes microseconds)
• Same things upvotes and downvotes as on landing page
• Primary buttons like "post" and "subscribe" could use some color to focus direction to them
• Community name in the sidenav could be a link to community
• What is this for I don't know. Do not display boxes with no content or show a label that there is no content
  
• View source button should be a button, not span
• Single word preview overflows
  
• IMHO post should be scoped below community in url, so you would know the community from the url (https://dev.lemmy.ml/post/29256 tells yo nothing about where you are), but that is little bit off topic
• When writing a reply, "Post" should have color to indicate primary action
• When seeing it for the first time on mobile I had issues separating the comments page from the sidenav and footer. Maybe some color separation? Maybe bigger margins?

Search page /search

• I would like to see quick search in navbar or some other global element
• Next button is visible before I have searched anything. It should only be visible if next page is actually available
• The form is not at all aligned or stretched. This is not consistent with other forms from the navbar
• There should be labels around dropdowns
  
• Add autofocus to the text input. Most common action will be - go to search, write stuff, press enter

Communities page

• Make subscribe be a button, not span
• It would be great if the whole row was clickable, not just the community name
• I don't really see a point of having the subscribe button on this page. Who is going to sub a community without getting looking at the content first anyway?
• On the other hand there are other interesting statistics about a community that could be shown. Like last post, posts per day, comments per day, posts/comments in last month and so on.
• Header sorting should have buttons, not just th
• There is no spinner when going to next page, but "prev" button appears before data is loaded