In Unix "less", can I jump to the next line that does NOT contain X?

Yes, this is non-match search functionality, for example:

less file.conf

then you type / and after that ! your last line should look like:

Non-match /

then type your pattern for example Non-match /^# to look for first line without beginning #

Instead of ! character you can also use Ctrl+N.

As an addition to Casual Coder's answer:

You could also filter out the lines altogether:

<logfile grep -v mypattern |less 

The disadvantage is that you have to quit and restart grep to change the pattern; the advantage is that the lines you don't want to see are hidden, which makes it easier to spot interesting patterns in the lines that you are interested in.

Not only can you jump between lines that don't contain X, you can hide the lines that do contain X, using less's & regex filter command.

less file.conf

then type &!^Computingthen the enter key and all those line will be hidden from view.

You can still search within the remaining lines with /.

Once you find the line you want you can bring the others back if you need their context - type & then the enter key to remove the filter.