I like trying different OSs, but I have yet to try a Illumos-based OS. Can I run all or most Linux Software, just by building from source?
You should be able to built most Linux software by hand (except very linux-kernel specific software), but as there are Illumos-based distributions which include package manager, it should be way more easy to use them and install pre-built software packages from the corresponding repositories. Using distributions with GNU userspace enhances also the compatibility with Linux/GNU system.
Building packages by hand can take very long and could require high system resources - if you are only trying some OSs, you normally do not want to do this.
No; you can try to build the software, but it doesn't mean it'll be a success without source modifications. For example; building PHP 5.5 on OmniOS will fail at socket compatibility requiring patches to be a success; you can see my modifications here to make it work.
Also, Linux based OS's are GNU; Illumos based ones are CDDL (the leaders of illumos-gate based projects prefer CDDL over GNU); they do come with GNU software but it's not the default. You'll find that
grep -F or certain
find functionality doesn't work on Illumos OS's unless you use the
/usr/gnu/bin version of the software.