For long documents, every copy-conscious designer uses InDesign's "Check Spelling" feature. I think we can all agree it can be improved; the functionality and interface have changed very little since InDesign's inception.

We need an "Ignore Capitalization" button/feature:

InDesign users spend most of their spell-check time clicking the "Skip" or "Ignore All" buttons: skipping capitalization issues (where there are forced line breaks or lead-in lines styled in all caps, making the uncapped offender a benign issue), skipping proper names, skipping the plural and possessive versions of words we've already confirmed as legitimate. Why aren't there buttons for "Ignore Capitalization" and "Ignore Case?"

We need a "(Go to) Previous" or "Back" button:

While we're on the topic of skipping, why isn't there a "Previous" or "Back" button? I'm not talking about the direction in which you chose to run a spell check in reference to your currently selected page (i.e. the "Forward" or "Backward" direction buttons); I'm talking about a button that—as you zone out on the endless repeat-clicks of the "Skip" or "Ignore All" button(s)—allows you to go back to a word you realize (too late) needs to be "Change[d]" and not "Skip[ped]."

Adobe's built-in dictionary needs an update:

The built-in dictionary is extremely outdated. Very few of the ubiquitous business-adopted words that originated in online communities and social media platforms have been added to the dictionary. Additionally, extremely common marketing and design terms are no where to be found. When I create marketing plans and proposals, this issue becomes acutely apparent. For instance, "online" isn't in the Adobe dictionary?! Seriously! If that's not a sign that the entire "Check Spelling" system is out of touch and out of date, I don't know what is.