SLIME emphasizes interactivity. Coupled with the fact that it uses the full power of Emacs (which is a small lisp vm, though a different dialect of lisp), you can do things like inspect any object or package in your system on the fly, view class hierarchies of your image, live documentation access, live disassembly of a function, incremental compilation into another buffer, arglist inspection, easily add amazing tools like paredit, autocompletion, inline macroexpansion utilities, source location lookup for functions and global variables, with Common Lisp spec lookup of any symbol all with with two keypresses or so. While many IDE's provide some or even all of these features, they are rarely as coherent and easy to use as they are in SLIME. Plus, SLIME itself is more easily customizable than something like Eclipse, because it uses elisp and common lisp as the backend language. A downside though, is that learning emacs is a prerequisite to using all the features of slime.