Given there's no tracking (feels a bit odd saying that nowadays with any kind of seriousness), do they monitor which queries people use !g for, and try to optimize their engine for these?
One feature I do like though (I'm not sure if Google etc offer this nowadays), is they'll often provide the top-answered StackOverflow answer in response to a query above the search results. That has proven handy a fair few times, esp. for some of the shorter answers (e.g. arcane command lines, git invocations etc).
Google does something like this when it shows images or scientific articles on top of other results.
DDG could even predict that it needs to fallback to Google in last resort.
DDG's auto question-answer stuff is pretty good. Not quite as polished as Google, but IMO far less annoying. Google thinks it knows better than I what I want. DDG seems willing to allow me to provide reasonable prompts. "Weather" and "what is..." queries, for example.
1. Older results. DDG's recent (<~5 year) results are pretty good. Going older than that for online content, not so much.
2. Books and journals. Google Books, Google Scholar, and Google Ngram Viewer are phenomenally useful tools, no doubt about it. There's no single resource which can substitute for them. Worldcat (library search) is good, if you've got access to various search databases (usually via an academic library) that's useful, and content access itself is possible (with questionable legality) via Sci-Hub and LibGen. The Internet Archive has less content, but greater legality.
3. Specific technical queries are generally less well answered by DDG, but easier to direct (via bang queries) to the appropriate source. StackExchange, Debian, Debian packages, Ubuntu, Microsoft KB, etc., are (I think) all covered. The ease of filtering from a general case to a specialised search is a real win.
4. Date-bound searches. I very frequently want to look for something between specific dates.
5. Less-prominent content. I use various sites (Ello, Reddit) as filing cabinets of a sort, and being able to search my own posts there is useful. Reddit's post search is good, Ello's not so much. DDG doesn't index (or retain) Ello results well.