I was so annoyed that I couldn't find it I opened a support ticket, they informed me to clear cache, internet files etc. Try a different browser. Yet they failed to mention that this was changed and now could only be found within a thread that you posted in. Consistency is one of my pet peeves when dealing with companies.
As a technician myself (different company, but tech support is tech support), I can assure you that this change was either A: Not communicated to the technician you dealt with or B: The technician is too lazy to read up on current issues.
It's most likely A, with the launch of MoP I imagine that the enterprises' resources were stretched a little thin, causing silly crap like this to fall through the cracks.
I'm still a little miffed by this change so I figured I'd come check out some replies, not surprised at all to see spam posts of people just posting to find their old posts, this was a guaranteed result of this change.
Blizzard, surely you're going to read this eventually so please, put that feature back. Even though not many have complained so far, for those of us who use the forums pretty frequently, this was a really nice feature that we lost.
I see that you don't have to spam a post to find your old posts, but really... come on... do you really want us to have to scroll down to the bottom to get this link? It's just unnecessary because you had a perfectly placed link before this change, in no way is this an improvement no matter how you look at it.
I know how easy it is to bookmark the all posts page, but that's not the point here. Your forums are the best I've ever used because it's feature-rich just like your games, but keep removing things that make is more accessible and it just creates needless complaints.