Platinum members ($50/year donation to SI) have a Follow feature - there's a little checkbox in the top right corner that says "Follow this thread". You can follow x number of threads...I forget exactly how many, and the least recently updated ones fall off in favor of newer ones as you add them, I believe.
There's more info in the Premium Access thread that you can see stickied at the top of the main forum page, the big red bar.
ETA Here is a link to the Premium Access information.
MH has added more features since it was rolled out, so the description on the first page doesn't include everything anymore, but if you read through the rest of the thread, MH updated every time a new feature was implemented.