Deletion of low quality content is a sticky wicket. I'm not sure that we have any clear guidelines, but your instincts are pretty much exactly what I would recommend.
If content is off-topic, spam, or offensive, you should feel free to vote to delete it. That's clearly content that we don't want on our site.
Unlike that type of content, low quality posts offer some content that could be potentially useful. It may be minimally useful, but our preference is to improve the post rather than delete it. For the examples listed above, improving the formatting, including a dictionary citation and definition, and leaving a comment to ask the poster to explain why they think that answer is a good fit for the question would all be helpful ways to improve the content. Another advantage of improving the posts you have noted here is that it will also help the users learn how to use the site.
In some cases though, you'll encounter some things that are low quality that really aren't worth trying to keep:
- no sense in trying to salvage an incomprehensible answer if comments had been posted asking for clarification and none had been given although [the user had returned since the comment was left or the comment was left weeks prior]
- the content is months or years old, the poster hasn't been back since it was posted, and there is significant question about what the intended answer was
- eliminating all the duplicated parts of an answer leaves not really much of anything else
It is a good idea to encourage users to improve their content, to help them do so, or to just do it yourself (especially for old stuff) in order to improve the content on the site. If a good faith effort has been made in this regard, and the post is still low quality, you should feel free to vote to delete it.