After reading the project page and getting through the slideshow I have got ... ehm, just few ... questions :D . Hopefully you will find some of them useful.
- How fast are they appearing / disappearing?
- Can you get more hovercards opened at once?
- Are the delays going to be changed in Preferences by users or hardcoded?
- Can it recognize (and show something different) you have already got that linked article in your Watchlist?
- Are hovercards showed only in main namespaces pages?
- Are hovercards showed only on links to main namespaces pages?
- Can it deal with links to chained redirects (page redirecting to other redirect)?
- Can it deal with links to redirects linking to other namespaces?
- How is "lead image" picked? Is it the first one in article?
- What about links to articles without images but with a (direct via template or indirect via their Wikidata item) connection to Commons category with some images?
- How do hovercards deal with large images (say, 1:10 panorama view)? Do they take the setting from the linked article?
- Is that "Add to Watchlist" star showed to not logged users too? I assume hovercards are showed to both logged and not logged users.
- How is the used length of snippet found?
- Are some parts (like other names, pseudonyms) from the top of the linked article ommited? I assume infoboxes and other templates like language ones or "add reference here" are not counted with .
- So the name of article is not showed in bold in hovercards? Are there going to be any other styles showed, like italics for, say, some music/literal pieces or just plain text?
- Are non breakable space taken from article or changed to normal ones?
- Are upper-case letters M and E with T in "Read More" and "Last Edited Today" really needed? Maybe even any such letters in those two messages.
- Is "Today" counted with UTC or user's system time (against UTC timestamp of edit)?
- Can it deal with "time ago" = 1?
- Can it deal with "time ago" = 0?
- How detailed is that "time ago"? For example, is "1h 13m" showed just as "1 hour ago" or "32s" showed as "1 minute ago"?
- Can it deal with "time ago" < 0?
- Will it even reflect edits to linked pages after this page was loaded? I assume yes as I see loading snippets for each linked page during loading of the current page would be reaaally slow.
- What about small "IPA" link in the corner for Wiktionary hovercards? With a link to some table with what-those-weird-sign-do-mean info (say, article IPA).
- Are there really links to Wiktionary in Wikipedia articles? Or how are those hovercards going to be picked? Are you going for "ok, this word is not a link to article, let's try looking it up on Wikitionary", so every word in text can possibly have its hovercard? That sounds like too much pop-ups.
- "Add audio prompt"? Will clicking on it actually take user to uploading area?
- "thanked for an edit"? What about linking the edit diff itself and not the article? Or is that too scary for common hovercard user?
- What about adding "Articles added to Watchlist via hovercards" to Analytics & Instrumentation section?
That's probably all ... for now.