Hello soapdog

what languages are available or if we're locked in C/C++ environment?

No. C/C++ is of course the most popular one, but a number of other languages have been ported (Python, Lua, Perl to name a few). There is also Amiga E (a general purpose language) and ARexx (targeted at srcipting and inter-application communication). If you want to search for some language try files section on http://morphos-news.de and also Aminet (http://main.aminet.net) may be worth to try.

How is GUI creation done in MOS and how well the libs behave?

MorphOS has MUI (Magic User Interface) built-in. MUI is object-oriented GUI system. There is no builder yet, but due to dynamic layout creating GUIs by hand is not difficult. MUI has some rare caveats but it is very stable and straightforward in general.

Is MorphOS a good system for shareware developers whose main focus is network applications?

I think so. We have BSD sockets here, so network programming is similar to other systems.

Is it fun to code for MOS or it's a nightmare

Difficult to say. It is fun for me, but I used to program for Amiga-like systems since 1997. It can be sometimes a nightmare (for example no source-level debugger).

can someone summarize things for me?

MOS Team was paid by Genesi until end of 2003, then Genesi decided to stop putting big money in the project. Now it is hobbyist OS with Genesi producing hardware able to run MOS (it can also run on old Amigas with PPC accelerators) and providing some non-financial support.