Unless a song is a solo instrument piece, all of the instruments and how they work together is very important. A great guitar riff, bass line, and drum line can still sound awful when put together. It is important to consider how these parts will go together and fit into the overall song. Sometimes it might be necessary to only have one instrument playing an interesting line while the rest of the band focuses on supporting that part. If everyone in the band is trying to show how good a musician they are, the songs may be too complex and contradictory. Good parts can still make bad songs.
Learning music theory makes songwriting faster, easier and allows the composer to make more complicated instrument interactions that still sound good. Music theory does not limit creativity, instead it usually heightens it. Rather than haphazardly trying to figure out what chords would make a good progression, someone with knowledge of theory already knows what chords should fit the key and scale of the song. They may still use chords that do not normally fit, but theory gives a good starting point before adding more atypical chord usage.
The dynamics to a song can reinforce and highlight certain sections of the song. This gives the song stronger emphasis in parts, such as the chorus, where the audience should be drawn further into the song. Using larger chord shapes with more notes, background singers, extra instrumentation or even just turning up the volume on electric instruments can all create a larger, grander feeling to part of the song. If the entire song is loud and large sounding, it will come off just as flat sounding and a more moderate or softer song without dynamic usage.
Taking ideas from other styles of music can result in some interesting-sounding songs. It is a good idea to listen to music outside of the typical genre using in your own songwriting. However, it is important not to lose sight of the main genre of your music or overuse the hybridized elements. There is a big difference between writing a rock song influenced by jazz and for a rock musician to just end up writing a jazz song. Be aware when hybridizing the techniques of another style of music that it does not take over the song completely.
One section of the song should flow into another. In addition, the verse and chorus will usually need to loop evenly on themselves. Sometimes, depending on the sections involved, the bar in a section that allows it to loop will not sound great when it comes time to move to another section of the song. Rather than rewriting the sections until they fit with one another, simply replacing that bar with a different one for the section shift will solve the problem. This can be the result of a dynamics change, chord switch or rhythm switch. Force fitting sections will almost always hurt a song.