Sorry for not specifying - if you hover over the bottom half of the link to a post, i.e. the part that shows Username, points, time since post submission, and read time, it will display "Show Highlight". Clicking on any part of the bottom half except the username will expand the item to show a section of the post, along with "Collapse" and "Continue to Full Post (59 words)" option (word count will vary; I used the one for this post as an example).
I prefer read time to word count because it brings it into normal/intuitive scale. Using word count is like measuring a human in inches rather than feet-and-inches - true and useful, but a bit harder to understand quickly.
On the other hand, some people may have better intuitions for word count than for read time. Perhaps an option to switch between word count and read time could be useful. This could naturally be extended to removing the length indicator entirely. I understand that it may not be feasible to add such an option; though.
I'm on the other side. I prefer word count to read time, because I know approximately how many words per minute I read. The read time calculation that LessWrong uses is approximately 300 words per minute. If you read faster or slower than that, the read times will be off for you.
This is more impactful for people who are slow readers; being told that something is a five minute read and finishing it in three minutes isn't a big deal. Being told that something is a five minute read and actually taking seven or eight minutes to finish is considerably worse. For this reason I would prefer word count to be the default.
Also, if you use the GreaterWrong viewer, you get the option to choose. You can click on the read time to switch it to word count. Clicking again switches it back.
Someone recently mentioned that they found the "read time" on home page post-items to be more of annoying clutter than a helpful indicator of whether to read something.
It occurred to me that I don't personally use them, and in general it's important for each UI element to be pulling it's weight. I'm curious how many people actively like/use them?
[Update: enough people have chimed in that I'm reasonably confident it's a good feature to have]