Tooltips seem to be pretty important on Stack Exchange. And it seems a lot of them get missed.
For example, the new mark your own question as dupe 'thing' lead to 2 questions being asked about that (here and here) - these wouldn't have been asked had it been clear there was a tooltip on the 'Community' user's link under the question. We know about this 'new' feature, but do new users know that? Probably not. (In case you have no idea what tooltip I'm talking about, rollover Community here):
Now, I'm not saying asking questions is a bad thing, but it does waste time for the asker and the dupe-marker, when all they really have to do is roll-over some text to get a pretty clear explanation.
Another example: all dates on SE have a tooltip saying their exact date and time that action happened - we know that, but do new users know that? Probably not.
Another example: Do you know how to see when a post you flagged was marked helpful? Did you find out by accident or did you think it was obvious? If it was the former, then this proposal will really help!
Another example: How do you find out the exact number of badges earned for a specific badge from the Badges page? Did you find out by accident or did you think it was obvious? If it was the former, then this proposal will really help!
Another example: How do you find out when a post was made community wiki without having to go to the posts' revisions?
What I propose
I recently asked a question on UX SE about how to tell people there is a tooltip somewhere for use on one of my websites, but I think the answers there are completely useable in SE!
Can we add a dotted underline under (important) text where there is a tooltip? This is pretty standard thing - sites use it to indicate rollover text on places. What's the point of having tooltips if their existence is unknown?
I think it will really help new users and existing users to find out things really easily and quickly. We could also change the cursor to a
? on hover (along with the dotted line) if you wanted to stretch it!
Here's what it would look like: