Well, I must say I haven't used audacity in a while.
Yes, it is free, it supports tons of different formats and already brings built in effects. the fact that brings "restoration" plugins (come on, not as flexible as cedar - ReTouch for instance - or even Izotope RX or waves, sonox...) it's a big plus: those plugins are usually expensive due to the algorithms used.
The fact that it's a very light software is not bad either - hence easy to fall in love and use. And it's alright for very simple projects, even serious ones that are quick and require just a handfull of operations. I could think of many podcasts, even editing jobs and quick rearranging.
Now, this is where it gets me:
No real time processing. Editing used to be a pain - at least for people that are used to a complete non-linear, non-destructive, instanced editing, and there were no tools to manage bigger projects, such as:
These are pretty much things that I couldn't live without on any musical or post-production project.
Also, no video playback support? :S
I miss stuff like comprehensive ruler editing tools (snap, grids, nudge, relative grids, snap to (aka spot), and other tools like a bit detective "smooth editor" for quick crossfading, TCE editing, maybe a smart tool?
Internal busses! needed for bounce, grouping, create send effects, even side-chain a compressor.
and, i know this might sound cheesy, but I do get a couple of these a year... Surround mixing? eh! :P