The only way i can describe it is similar to facebook. Many times i post on threads asking questions, then come back to it the next day and have to search through to find the thread, then my post. And sometimes there isnt even a reply. But, a notification alert, that took you straight to the thead/post would be oh so handy
I've been thinking this quite a while now
"Spin the middle side topwise. Topwise!"

"And there's Jimmy Page, the biggest thief of American Blues music"
If you open your last posts on your profile, you can see if somebody responded on a thread since last time you checked; if nobody replied, the envelope icon will be closed; if somebody replied, the envelope will be open.

There is no need for such notifications, as they will only bring spam to overly active UG-ers (just like facebook does).
Quote by ChemicalFire
You get my first ever lolstack

The image in my head is just too funny for words at this point

Aw yeah.
You can enable notifications for new responses in the Thread. To enable this option, press the link "Subscribe to this Thread" below Quick Reply window and select Notification Type.
Also, you can check if anybody has quoted any of your posts in the 'Search' function.

Type in your username as the keyword, and show results as posts.
I'd love something that showed up in UG instead of emailing it. If I'm bouncing in between a dozen threads in 4-5 subforums I don't really want to have to check my email. If something would just pop up on the UG page (like that PM notification on the top right) it would make responding a lot quicker and easier.

The email notifier is too cumbersome to use on a day-to-day basis.