I'm a Guide on my main, and it's pretty interesting to see what questions pop up in Newcomer chat. The most common thing people seem to be having trouble with is using emotes. Or possibly that's just a mistake which ends up in chat, while other mistakes are invisible. The other major mistake, which is also visible, is talking in the wrong channel.
For background, one of the quests in the new introductory area of Exile's Reach asks you to /wave to an ogre (and then to /dance with him). It's very common to see newbies type "wave" or "wave to Gor'goroth" in newcomer chat (which appears to be the default for new players).
Slash commands are second nature to experienced players, so it's enlightening to see how foreign they are at first. It's possibly even more foreign nowadays in the age of phone and tablet apps, where everything is done through taps and gestures.
I wonder if WoW should follow FFXIV's lead and create an emote menu, with icons you can drag to your bars. You can do this now with macros (memories of /kneel macros to get the achievement for Fandral Staghelm in Firelands), but you can't expect a beginner to handle macros.
I do like WoW's implementation of the guide system a little bit better than FFXIV's mentor system. There's no visible marker for guides, unlike the crown in FFXIV. Guides can identify new players (murloc marker), and new players can identify guides (green flag marker), and both have access to Newcomer chat, but that's it. As a result, I think Newcomer chat is pretty helpful and sensible.