Yes, custom classification/tagging/labeling of transactions as well as classification/tagging/labeling of invoices, jobs, etc is a really important feature.
This classification should ideally enable a hirerarchy, as Ben Bucksch in his post to the original feature request wrote. This enables to analyse/report expenses per Program>Project>ProjectTeam or per a hierarchy of cost centres or per a hierarchy of customers or whatever else like this.
Enable putting the transaction into zero, one or more such a custom classes.

Please let me distinguish such a "transaction classification" feature as special case of "user defined fields/attributes" which I describe in a separate request:
http://gnucash.uservoice.com/forums/101223-feature-request/suggestions/1951947-user-defined-fields-attributes?ref=title

I would like to see this for each transaction split, not only one tag per transaction.

The existence and popularity of Quicken's classes shows that there's demand for this facility.

Something like a tag system would be very powerfull and simple to implement. One should be able to group a transaction under zero, one or many tags just by adding it at the description field or at any other specific field. With this, all the existing structure remains the same, but we can have new reports filtered by these tags and/or accounts.

As a followup to my previous post--after searching for transactions you can then go to Reports/Account Report to get a report showing only those transactions that were found by the search.

I have wanted this feature for years. Tags, labels, or Quicken-like classes--any of these would do. As others have noted, it could greatly reduce account structure complexity.

I have found a work-around for simple projects: develop your own tag system and put the tag as the first part of the transaction description. Very simple reports can then be generated by using Gnucash's transaction search feature and searching for that tag.

Please see image for similar functionality
http://financessoftware.com/assets/gallery/7.png.

Although the Quicken "classes" nomenclature seems to be unpopular, that is the functionality that is really needed. In more robust accounting programs it is sometimes referred to as project cost accounting. I can just as easily pull all income and expenses per project, as get a total for all expenditure x across all projects. So far I see no way to do this in GnuCash. Would be VERY welcome.

Some kind of labels would be useful to easily group transactions. Of course label functionality can be achieved by proper account structure but sometimes it isn't worth the work needed.

BTW, in the example below I would prefer delegation over inheritance ;) and create virtual accounts:

Properties:Property1
Properties:Property2

Then I would transfer from Cash to Electricity through Properties. Every property would be balanced to 0 but have all information about transactions related to this property. (This is similar to example from bugzilla link but transactions in Properties has values.)

I would like to see a separate category system like "tags" found at www.mint.com or "labels" found at www.gmail.com

This would reduce the need for complex account structures and allow for a different dimension of filtering and reporting.

You could tag certain transactions for tax purposes, reimbursements, or family member names.

The challenge will be of course in the filtering and reporting.

As a pointer to previous discussion: https://bugzilla.gnome.org/show_bug.cgi?id=113772