I do think that Dannae presents a valid (and funny) point in her example. That's something that you see come up lots of times, especially when it comes to RP-fighting. Aramara also makes a good point about this, here:
most people do break their dialogue and actions up into separate posts, which is how i used to play as well. But that creates a problem where you're never sure when that player is 'done'.
The uncertainty of whether to proceed with a RP or not can be hindering to the general flow of the storyline, itself. Whether or not the ability to tag in /me and /my into a sentence will ultimately fix this problem is uncertain, but I do feel confident that it would certainly help address the issue.
Maybe if you want to split your comment up put at the end of your comment in brackets [to be continued] or [tbc].
I like Bonifarzia's idea on the /narative.
This is waaaay off topic but, with RP what I find holds me back and is a main reason why most of the time I don't really enjoy RPing is the chat tab in general. It's very annoying and basic, the selecting words and copy/paste would be beneficial. But what about the option for RPers to have the whole of the bottom screen the box to type your comment (like
http://webchat.freenode.net/) and have the chat tab above it to the left or right of the screen. The availability to have 2 or 3 or 4 or however many lines, so your text will pop down to the next line until you reach your chosen cap and then your text starts to disappear off the side. (I hope that made sense).