How do I get that nice inline-quote indicator in Outlook 2010 when replying to emails?

1. Click the File tab

2. Choose Options from the left menu

3. Choose Mail from the left menu

4. Scroll down for Replies and Forwards

5. Choose your relevant options from the dropdown. I think what you're looking for is "Prefix each line of the original message"

To remove the existing prefix / blue bar, press Ctrl-Q (per @djp; be sure to up-vote that comment below if this helps).

As the comments have noted is an issue with variant's answer, you can't add text into the reply if you use the "Prefix each line of the original message" feature.

This is what I'm doing that takes a bit more setup and a bit more work to use but is way more flexible. Note that I'm using Office 365 ProPlus

1. Double click on some text to highlight it then click on the Styles menu
2. Name your new style. Click the Modify button
3. Make sure the checkbox Add to the Styles gallery is checked.
4. Make sure the option Style for following paragraph: is set to ¶ Normal
5. Click the Format drop down then click on Border
6. Set a left border, with the color you want, and the width you want.
7. Keep clicking Ok until you're back to your e-mail

Now that you've got your reply text style created, just highlight text that you want to mark and apply the style to your replies. Of course, you can apply all kinds of other customization to the style as you see fit.