Let me qualify what I say with this - I've never written a song and it's not something I really desire to do at the moment. It's challenging enough trying to play what somebody else wrote :-).

If I were to attempt it though, I would start off with what appeals to me and forget what might appeal to others and what the current conventions are regarding song length and style. Being a fan of progressive rock and jazz, I like solos and longer songs. Any song I wrote would have to feature a kick ass guitar solo :-).

Do you like power chords and the way they sound? Do they inspire you? If so, use them! Barre chords more your style? Use them instead. Think about the ballads that you like and what it is about them that you like...incoprate that.

Once you have that ball rolling...go with where it takes you.

As far as soloing - in my messing around, I find I can use minor pentatonics over just about any chord sequence. You can play notes from a different scale over each chord (Gilmour does this), but you don't have to.

When you are noodling around and experimenting, have a recorder going all of the time. I wish I did this. Many times I have stumbled upon what seemed like a cool riff out of pure inspiration of the moment only to forget it shortly thereafter.

Muscians write songs without bands all of the time. Pete Townsend would put together demos in his home studio all of the time before presenting them to the rest of the band to be fleshed out. Some of these demos would just be him singing and strumming an acoustic guitar.