The main problem is that it's boring and flat. It's actually really hard to make objects, especially manufactured ones, not look boring. The usual process for a newer artist is to think of some object of interest, draw some lines in a perfect side/isometric view, then fill it in with colors. They look at it and see something is not quite right, it's very unimpressive. So, they add some shading in, usually around the edges, but still without skill they do so timidly and don't try anything extreme or crazy, just a slight hue shift hugging the lines. Then, it still looks a bit boring so they add on some details, like windows or torpedo chutes or whatever.
Well that is a process everyone goes through, but I'll tell you that the reason it looks plain is not because there is too little detail, but because you are approaching it in fundamentally the wrong way. Consider the difference between this and a submarine that a really good artist would draw. Those differences are what you should consider, and what you should pursue if you want to improve. Here are a few things that might point you in a vaguely correct direction:
- Does the perspective need to be a perfect side view? Side scrolling games don't just get away with breaking the plane, they rely on it to look good.
- Is such a large plane of one single color justifiable? A submarine is a cylindrical object, with no light coming from below it, and different amounts coming from above. The shading can and should be much more prominent, to the point that the base of the object is the shading, not the lines. Try and draw the submarine with 3 or so shades of grey, using no lines, as a challenge. You may be confused on how to approach it, but in the case just look at pictures of real submarines. Then, just start hacking away at it. Use a thick brush and just try and get a rough approximation, be messy.
- Are your color choices well thought out, or are they just simple variations on the "main" color? Light is a funny thing, it almost always changes the color (hue) of what it hits, depending on how it hits it, or how much/little hits it, or what else is in the environment, or what kind of light it is, etc... Consider that some colors are inherently more "light" then others because of this. Yellow for instance is very prominent where light is hitting something strongly. Purple is prominent where light is not hitting something strongly. Use this fact in your color choices to bring things to life.
- Consider the background you are drawing it on. I know it's going to be keyed out in the game, but you should think about how the sub is going to look in the environment that is will exist, which I doubt will be pure white.
Hope it helps some