I see that you changed the title of your topic to gather more constructive feedback.
First off, I'm pretty new to digital art so I'm learning every time I put pen to tablet.

From you Deviant Art page, it looks like you've experimented with different brushes and techniques; that's a good thing. I like how you have less reliance on the airbrush tools now. Too much airbrushing looks undisciplined to me.
I like your scribbled style and the overall proportions you use for your characters. A little more attention to symmetrical proportions would be better. The girl with the blue/gray hair above has a really short left arm, for example. However, it doesn't detract from the piece too much so you're obviously doing something right.
I'm guilty of this as well, but you haven't drawn much outside your comfort zone. I really like your cute paper doll characters, but eventually you're going to have to draw them in different poses and different angles, interacting with one another and such.
You're noticeably consistent though. When I see amateur art, I usually cringe, but your stuff is really attractive.
Can't wait to see something Aquaria inspired.

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Have you tried MyPaint and GIMP? I noticed that you use SAI and was just curious how it compares to the aforementioned tools.