Having been in your boat not long ago, I know exactly what you're looking for - constructive and honest criticism. So please don't think me mean or anything, because I understand what you mean when you say all you have received are "good job, keep up the good work" which really doesn't help at all.
So here we go... my honest opinion as a graphic designer and RL Architect.
GRAPHICS:
The Issue:
Graphically, I feel like your banner and thumbnails are lacking in visual appeal and consistency. They have the appearance of being made by someone who just got Photoshop and thinks bevel + 5 pt stroke is cool. When you're doing graphics for anything, you don't want people to be able to figure out exactly how you did what you did - because if they can right away, it shows a lack of effort on the part of the graphic designer. Now, don't misunderstand me, I don't mean to say you didn't try at all, this is just the feeling graphic designers like myself will get when I see your channel art.
Your thumbnails, although they are consistent in that they use the same font (a good thing), they don't speak to me as uniquely branded as something from your channel. The text is arranged randomly, at different sizes. It is a simple stock font available to all PC users, white in color and with a small black stroke.
The Solution:
Figure out a logo, which is a great way to brand yourself. It will make you immediately recognizable, and should appear almost everywhere. This includes your banner and thumbnails. Check out some tutorials on Photoshop for doing some cooler things with text and graphics. In addition, consider locating a custom font, there are many resouces on the internet where you can find these. I suggest Abstract Fonts, they always have some cool stuff.
AUDIO:
The Issue:
Your stock Razer headset microphone has extremely limited range in frequencies, and the diaphragm size causes nasty distortion when you start pushing it to the limit in volume (shouting). On the plus side though, I haven't heard much background noise - which is fantastic. That doesn't mean you shouldn't use a noise gate or EQ in post production though, since adding those in will give your audio a new level of polish. You can generally make your microphone sound substantially better with even just minimal well directed post production EQ. Check the internet for tutorials on such things.
The Solution:
Since you seem to be serious about pursuing this as a hobby, seriously consider purchasing a higher end stand alone microphone. You have several options for this, such as a condenser microphone, or a cardioid microphone. For decent mics on a budget, look to AKG Precision series condensers, Blue Yeti, or my personal choice, the Shure SM7B cardioid dynamic microphone. Condensers are more finicky if you go that route, and can be more easily broken by standard ware and tear, or just by being left plugged in. They also tend to pick up more background noise. Despite this, condensers yield the best post recorded audio possible, and are often used in music and movies for audio recording. Dynamics give you a more robust microphone, capable of being dealt pretty hefty blows - great for travel or extended use. They also pick up less background noise as they are directional, and generally sound deeper and more woolly, great for radio, youtube, and podcasting.
In addition to a mic, consider an external usb or analog mixer, they generally run pretty cheap (60$ USD) and will allow you to change audio channels and adjust EQ on the fly. I use a Xenyx 802 analog with British style EQ (High, Mid, Low freq adjustment), with the low end dropped down so my mic sounds nice and bassy with my voice. You also NEED one with phantom power (3 pronged audio cable interface) if you go the route of higher end microphones.
Finally, find some tutorials on good EQ filters to drop on your audio in post. Once you set it up once, you can just drop it down on your audio every time in post production to yield the same, high end feeling results.
I know I didn't go into marketing, but that's something you will want to do AFTER you've done some more polishing.
All for now, wish you the best of luck!
