I always get a bit nervous when I answer what seems to be such an easy question, because I start to think I must be missing something. Sometimes, there's good reason to be nervous. This is one of those times! Answering your "simple" question exposed a gap in my understanding, which is that items in a project that is "on hold" or "dropped" will not show up in context mode, no matter what you do. For some reason, that had escaped me completely, and I'm glad you asked the question!
I tend not to put projects with even the slightest trace of activity on hold, so my usage is a bit different than yours. I don't think I've ever put a project on hold when waiting for someone or something. If I need to be sure that a wait will be limited before following up, I'll put a due date on the waiting for action.
I'm assuming in my answer that your "waiting fors" are contexts which are marked "on hold" (as are mine). I'm also assuming that you aren't putting the projects with "waiting fors" on hold, as mentioned above.
In context mode, turn on the view bar and put in these settings:
In the sidebar, select all of your "waiting for" contexts (which should be the only ones in the list, unless you have others which are also marked "on hold"). The content shown in the main pane should show all the actions which are causing projects to stall. However, it will also show the "waiting for" actions in projects where you haven't gotten to the "waiting for" action(s). For example, a project where you have to make a decision, then order something, then wait for the delivery will cause the wait for delivery step to appear in this list even though you might not yet have ordered. We have to select Remaining actions to pick up the "on hold" actions, because there is no "actions which would be available but aren't because they are on hold" filter. I personally don't find this to be too much of a problem because my projects usually go quickly up to the point where I'm waiting, and I often don't plan them out far enough that I need to insert a "waiting for" until, well, I'm waiting for something :-) If you want to examine all of the projects with "waiting for" actions to make sure that there isn't something else you should be done, you can do the following (which looks much worse than it is to do):
- bring up the previously mentioned settings (in context mode)
- click in the right hand pane on a project name, then do cmd-A or File->Select All
- switch from context mode to project mode (cmd-opt-R or View->Show in Planning Mode)
- focus on the selected items (cmd-cntrl-F, View->Focus on ..., Focus button in toolbar, or use the popup menu gotten by control-clicking in the sidebar)
- set the view filter to show remaining actions
- look at the resulting list of projects/actions
If you've done all of that without changing the selection by accident, your "waiting for" items will all be in the selected color scheme, and you can easily see if they are the next action for the project(s) they occupy, and if not, what remains to be done first. The following example shows a mix of possibilities.