The pen tool can get really cool results, I wouldn't write it off immediately. If the way you use it is just Pen>create stroke along Pen then you wont make an image look as nice. You should try to make the lines by using two lines from a pen and just filling them them in so you have total control over the line thickness and what not, I think Psi's art used the pen tool exclusively so if you dont believe me then look at his art, it shows that pen tool is super viable.
AS for the criticism I swear I should just have a copy pasta to post every time someone asks this because I find that I always say the same thing, mostly because what I say i think is what I would have told younger me when he was getting into art.
Ill just try to summarize as best I can since I usually end up in a huge rant.
FIRST, always draw from real life. Still lifes and figure drawings, you need to get a good grasp on anatomy and how lighting works in real life, then you can stylize it in your cartoony drawings properly.
SECOND look up tutorials on dA or tumblr and what not. Stuff like HOW TO DRAW HAIR, HOW TO DRAW EARS, HOW TO DRAW THE MIDDLE TOE, HOW TO DRAW GRASS, yes, even grass, learning any concepts like how to draw trees, grass, armor, will help you. As an artist you can be happy with just knowing how to draw hot nudes, you need to know how to draw the bed that nude is on, the pendulum the nude is staring at and the grass that the nude is lying in.
THIRD take all critical feedback you can, the only way to get better is to know what youre doing wrong, you dont need to be an artist to see if something looks wrong.
FOURTH you wanna draw pinups and hot newds? learn Composition, gesture drawing, line of action and perspective. You gotta know how to make an interesting picture using the human body and only the human body. Doing a pose where someone is sitting on the a bed touching boob is fine, but why not make it a POV where the girl is leaning in facing you on a bed with one of her hands reaching out to you while the other reaches down and plays with herself.
FIVE do studies, sit down, and draw hands 20 times, then faces, then eyes, just draw, draw drawdrawdraw, wanna get gud? drawdrawdraw
2 channels i recommend to everyone
www.youtube.com/user/Sycrawww.youtube.com/user/ProkoTVwatch some of their tutorials, they are super basic and fundamental. they helped me ALOT they can help you
Ctrlpaint is also really good, they taught me how to do lighting properly like ambient shadows, rim lighting, directional shadows, etc, but they are really good for overall digital art
there was a thread recently with someone looking for art advice, i pretty much said the same thing there too but its worth looking at
hypnohub.net/forum/show/15581 i do mention stuff about developing your own style in there too if you want to skim that
also luna mentioned using multiply for shading and overlay for lighting, and its not TOTALLY wrong, I used to do that and sometimes still do for shading but what more important is to understand color theory and lighting properly. You need to know what color the shadow should be, how dark is it, is it a soft shadow? is it a hard one? how far is it from the source, how blurry does it get, how hard should the light be, where is the light coming from, how much of the light washes out the color. etc. Looking at your art you should really practice this, you seem to be falling into the PILLOW SHADING trap.
Please google PILLOW SHADING, you'll understand what I mean immediately, i hope.
NOW FOR YOUR ART SPECIFICALLY
the first thing that stood out to me is your figures are all really blobby. I may sound a little mean, and I don't want you taking it the wrong way, critiques can be rough and im kind of a dick sometimes so bear with me. I kind of felt like the body's style and proportions are based off of teletubies, everything seems chubby, soft and rounded where it shouldnt be. Also the way you draw faces seems far too simplified, you need to draw real life faces before you start doing stylized stuff, gotta know the rules before you break them. Your lines are also a tad shaky, the way i do it is draw with quick long strokes, avoid chicken scratching when you draw lines, thats fine for sketches but unless you have a edgy rough style its not gonna work out. And dont trace over them slowly either cuz thats why they come out jagged. Overall all i can say is just draw real life figures and follow as many tutorials as you can. Watch other people draw, speedpaints are okay but they can be intimidating because someone draws something masterfully so quickly and you barely even see how, livestreams and real time drawing is better imo so you can see what the person is doing. Your shading is all really soft too, you need to know where a shadow should have a hard edge and where it should be soft, and again, you learn that from real life drawing.
I have a lot more i can say but those seem to be the dire points, the first few months drawing are the hardest but they are also when you improve the fastest. once youve been drawing for a long time its harder to improve, at least for me so bear with it, all those amazing hot girls people draw were drawn by someone who drew pointy chin giant eye'd dA tier art at one point so dont be discouraged.
Anyway I ended up ranting a real long on AGIAIN GODAMNITWHY DO I KEEP DOING THIS I hope this helps you a bit. I know of some artist who when they got some criticism for their art instantly call those people haters and tell them if you dont like it ignore it. and THATS BAD, WRONG, STUPID AND IF YOU DO THAT I PITY THE HELL OUT OF YOU. Good luck on your art things. Also I've done commissions before and I'm currently doing game dev stuff myself at school so I can tell you that you have a long way to go, im saying this not to discourage but hopefully to motivate you to get good and get good faster, whenever i see an artist who is better than me and at my age, my first thought is I'M GOING TO BE BETTER THAN YOU AND I WILL WORK SO HARD UNTIL I ACHIEVE MY GOAL. I say that is a good way to go about it.