[tsundoku]

I had the same question (how do I export a list of all my tags?) and found some long threads on it. When I went to add how I did it to the thread, the Mobile Read UI suggested I create a new thread. So here are step-by-step instruction for how I did it in case it's helpful:

In Calibre
- Add Books | Add Empty Book
- Select the Empty Book
- Edit Metadata
- Open Tag Editor (button to the left of the Tags dropdown control)
- Drag-select all the tags by holding down the CTRL key (in Windows)
- Add all the tags by pressing the “>” button
- Double-click on the tags control to select all the tags
- Ctrl-C to copy them

In Excel
- Select an empty cell
- CTRL-V to paste the list of tags into a single cell
- Data | Text to Columns
- Choose Delimited, Next
- Select Comma, Unselect Tab, Next
- Finish (you now have each tag in its own cell/column)
- CTRL-SHIFT-Right Arrow (to select all the columns with tags)
- CTRL-C to copy
- Select a blank cell
- Home | Paste | Transpose

You now have your list of tags in a single column in Excel.