1) Basic functional search
Search should handle different order of words, misspellings (fuzziness), offer indexing and searching in a larger scope than just titles and aliases (e.g. headers or content), as well as allowing users to customize search priorities. Basically - just include Omnisearch as a core plugin.
2) Basic image preview
Displaying an image on full screen, with panning and zoom, when clicked upon.
3) Full "folder notes" support
Out-of-the-box support for a vault structure where each note has its own dedicated folder where all its attachments are placed. While the basic functionality is present, an external plugin is required to declutter the vault file hierarchy and actually make this approach feasible. Folder notes approach is in my opinion the only way to keep a large vault organized.
4) Basic formatting.
Text coloring. Text alignment and justification. Basic image positioning. Proper text flow wrapping around images. Table formatting (at least a setting minimum column width).
5) Markdown parsing within HTML tags
Basics Markdown features like [[linking]] don't work within a section of text enclosed by HTML tags. And using HTML/CSS is currently required to achieve basic formatting like centered or colored text.
6) Option to use the first h1 tag as the note title
I'm talking about actual support for this and integration with core functionality like search and linking. Useful (sometimes long) titles are an essential part of note-taking and knowledge databases. Meanwhile, filenames are simply semi-unique file system identifiers. Forcing users to use filenames as titles compromises the usefulness of titles and leads to issues with filename / filepath length. In HTML and Markdown, the h1 tag was always intended for the title.
7) Consistent formatting between reading view and editing view
Rendering of content, especially vertical spacing between elements differs between those views for no credible reason. The code syntax highlighter is also deficient in editing mode, despite it being the mode in which Obsidian users spend 99% of their time while writing, editing and reviewing notes.
It's not an exhaustive list, but these are the biggest pain points right now. And let me repeat - you shouldn't continue to rely on community plugins for these features. Even though community plugins are great, they are a security concern, their development could cease at any point, and new users don't know about them.