#+ is only one of many, many standard reader macros (as you can see from that link). You can also define your own, which makes Lisp syntax almost infinitely malleable. For example, it's possible to change the Lisp reader so that it natively reads infix syntax. See:

http://flownet.com/ron/lisp/djbec.lisp

for an example. Scroll down to the definition of xpt-add.