Duplicate code among install/interface/server
There is currently important code which is duplicated among the different parts of ISPConfig (i.e. install, interface and server). I think of file manipulation functions, getconf, distro detection, and more...
I guess the installer could take care to duplicate that code at installation time, if no better solution (ex.: common directory with those libs) is felt applicable.
I think I don't have to explain why code duplication is bad (ex.: maintenance, disk space, memory usage, ...). I'm willing to help any real PHP coder (I'm not) to work on that.