This is just a list to brainstorm possible improvements to the currently very basic feature.

Functionality:

• Print memory usage
• Filter and show messages by category only
• Show in separate window OR add a GUI browser to read debug messages more easily
• Copy & Paste from a new window

Formatting:

• Better documentation

Optimization

• Performance? Memory footprint, profiling impact... maybe add a toggle to disable the queue entirely? Ideal default cache size?
• Using tremove isn't efficient for more than a few hundred messages... should be okay, though?
• Use = nil instead of tremove (recommended by Lua authors)
• No need to set and store the default category. In fact,
• Don't save redundant data; but store categories by index/name instead?

Addon-specific

• Store last debug log in saved variables (meh) - would easier to copy/paste, but I don't like it
• Add more slash commands
• Add usage to README and FAQ?
• Add setting to config GUI
• Disable in release versions? (kind of defeats the point... maybe disable in release and enable for alpha/beta if there is a setting?)
• Move reason from debug msg text to category field
• Rework slash command dump - variable number of messages

I don't even remember what half of these were about. But here's my updated evaluation of the main concerns.

Memory usage

• The cache currently stores 250 messages (lines), which would be 256 characters per line in the worst case
• So that's 256*250 = 62 KB plus some bookkeeping / table metadata and other overhead
• That is to say, the increased memory usage for this rather useful feature seems more than acceptable
• It's still possible to limit the cache size or even disable it altogether, albeit not exposed to the UI currently

Usability

• Debug messages being output to the default ChatFrame is awkward from a UX perspective
• It requires people to take screenshots to report issues, which is impractical and not searchable either
• Filtering by category would be neat, but there's only Debug and Profiling right now - so that's a meh at best
• Copy/paste is a great improvement and I have something working (prototype, needs cleanup)

Documentation

Definitely should be documented better since no one who wants to report an issue knows of this feature.