What's wrong with WotLK that would indicate a decline in the famous Blizzard polish/quality?
Beyond simply being released too early - the last few patches as well as the expansion has had more technical difficulties and much more blatant balancing issues than what used to be the norm.
Beta testers were being very loud about a multitude of issues before launch - and I think many of us were surprised that it was released when it was. I expected at least 2-3 months of additional polish and balancing.
I think this has been the case for quite a while with WoW - and I seem to recall them having a much clearer vision of classes and PvE/PvP balancing during the first couple of years.
Not that balance wasn't always a concern, but it certainly seemed like they took it a lot more seriously pre-WotLK.
Also, and I'm not sure they fixed it yet since I haven't played since the first month of WotLK release, but there was a VERY serious bug with players using mouse-look. They would pop around on the screen as if they were affected by some very serious lag, and beyond being merely incredibly annoying - it was a problem in PvP because you couldn't be too sure where they were. People weren't really registering this until they were made aware of it, because they assumed it was normal lag - especially in conjunction with launch.
A bunch of players reported this with strong evidence - including myself - and AFAIK they didn't even acknowledge this and certainly ignored it for a couple of months. I doubt they've addressed it yet.
Such a thing I wouldn't have expected of Blizzard a few years ago.