1) No matter what DAW you use, they're all essentially the same. If you're bored with the FL Studio interface, maybe you can change your background and move some shit around, but honestly it's the same shit
http://www.kaz.ph/blog/2012/12/06/customize-fl-studio/
you could go through the effort of trying to learn a new program, but it really won't make a significant impact on the output of your product
2) Download some VST's. You can find plenty of free ones on KVRAudio.com - or you can bootleg the free ones (which are generally better). But there's an endless supply of Sounds, Reverbs, EQ's, Distortions, Flangers, Choruses, etc.
3) I typically come up with the chord progressions before the melody. I feel like it's a lot easier to make a melody that goes well with a chord progression than the other way around. The concept behind constructing melodies is usually a "journey" in the sense that a melody starts somewhere, then finally reaches it's ultimate definition before going back home. If it's something fast-paced, I'll usually use more skips and leaps to keep it interesting - if it's something slow or emotional, I tend to use more steps
4) Mixing is 75% of the battle. My computer doesn't quite handle FL Studio as well when I've got a shit ton of distortions, reverbs, and EQ's all going - so I tend to do some of my mixing in Reaper. Just mix, mix, and mix some more.
Yeezus is actually a great example of a lot of really shitty, simplistic beats that were so well mixed that they sound amazing.