Ascending = Oldest First

e.g. reading forum topics or blogs follow the story, comments, replies to the end rather than starting at the end

however the active page here presents the alternative as it should be newest first, recent posts/comments, news stories should be newest

the only thing I usually want to sort descending by default, against the grain of tabular data (spreadsheets) is financial/numerical data where only the end figure is important anyway

Some data needs to be read in order. That's Ascending.

Some data has its history rendered void by current data. That's Descending.

Unfortunately, most logfiles are ascending, and they should be descending.

I get peeved at discussion forums that insist on showing you posts in descending order, so you are looking at a reply with no context.

Blog posts: Newest first
Blog comments: Oldest first
Forum threads: Last modified first
Forum posts: Oldest first
Shops: Bestsellers first
Search engines: Best matching the query first
Logfiles: Newest first

In general, if there is a natural order in the items which increases readibilty if that order is followed: "oldest first". If items are not related: "most wanted first" where "most wanted" can have different meanings dependent on the context.

As already said different data have different natural orderings. In all cases the visitor should be able to reverse the order and ideally sort by other parameters.