The user interface, it's simple enough that even I can use without any training or help from developers / support staff yet powerful at same time! It provides such great flexibility in terms of customisation that you could really do anything with tracs - but this comes back later when trying get some things done as there are just too many options out there (i have seen people using 100+ plugins!).
This makes getting your desired functionality very difficult compared to other tools which offer fewer choices/options. Also since its open source no one else will be able solve problems like yours if they don't know how because someone has already solved them before :D. We used Trac mainly so we could track our own workflows internally however now everyone who works here knows exactly where i am up to within seconds :).