(Copy/Pasted from "Lasse V. Karlsen"'s answer)
You want inhouse developers and testers to take any and all things they notice in the software and plug it into the tool, even if they're currently working on something else. For this to happen, the tool must be so easy to use that it stays out of the way and just takes your data. The worst bugs are those you don't know about.
(End of Copy/Paste)
Even good, conscientious testers, if they are focused on testing component A but happened to stumble on a bug in component B, might not actually enter that bug if there is a lot of friction in the bug tracker. Friction means, required fields. It's not that the testers are bad or lazy - it's just how the human mind works. We focus. We don't see the guy in the gorilla suit.
The Joel/FogBugz philosophy of NO required fields is the right one (Also the philosophy of my own BugTracker.NET). You almost always can gather the details later - what os, what version, what browser, etc.
Also, take a look at "Bug Shooting", if your app has a GUI. You want to make it as easy as possible for the testers to take a screenshot and get it into the bug tracker, and that's a great tool for it. Pick a tracker that works with Bug Shooting or has its own dedicated screen shot tool.