Good work for a beginner. I like your effort.
First off, I'm taking out some time to make comments on your animations, as you've personally requested me to do so. So, please take them constructively.
1) Horse Gallop: Its fine, but its weightless. It kinda looks like a flying horse, who is not touching the ground much. Thats because, the upper part of the body is totally stiff... especially the shoulders of the horse, the neck, and the head. You have to follow the principles of lead and follow. You can take the pelvis as the center of the body and let the outer most part follow it. For example in a biped, it should be like (most of the time), pelvis leads and the lower abdomen follows it. Then the upper abdomen follows the lower one, then the lower chest follows the upper abdomen, and the upper chest follows the lower chest, then the neck follows the upper chest and the head follows the neck. Its like a chain reaction. The pelvis (center) leads and the outer parts of the body follows it. Go ahead and get used to some concepts of overlaps and follow through (do simple test animations!) and then come back later to work on these types of exercises.
2) Dog animation: It looks better, maybe because its a small animal which you're animating. But the above rules will apply here as well. Body is a single cohesive unit and you have to make every part of the body work together.
3) A Guy running: Learn the basics of walk and run, and stop using interpolation for the sake of learning. You're giving the control to your computer which makes it floaty. The problem is not the computer, the problem is you're not sure how to use the computer to help you with your animation. You're simply surrendering most of your stuff to the computer. I would recommend you that you animate everything in stepped tangents and keep them in 2s' (just for the sake of learning the basics). Or in other words, you have to take responsibility of every alternate frames! This is just what I would recommend you.
Most of the CG anim noobs I've met, avoid working in 2D, as they are totally unaware of the power of 2D. Do a lot of drawing/sketching, learn gesture drawings and use them in your 2d animation. With time, your animations will become strong as hell.
SpaceShip anim: Avoid these space ship type animations in your in your character animation demo reel. They hardly show any of your animating skills.
Lifting weight: Just lifting an object slowly doesn't help much to create an illusion of weight. Learn about posing, line of action, silhouettes and also, make several gesture drawings of a person lifting a heavy block or an object. Capture ONE single frame of him lifting the weight off the ground. That single frame or drawing should have a lot of weight in it. You are free to exaggerate the pose or caricature it.
These are some of the stuff which applies to all of your animations.
The guy lifting a small box: This animation is very weak. Everything (the box, etc) floating around. You don't have to show this animation in your reel. Just get rid of it.
A few words of wisdom from me
Stop focusing on demo reels and love what you're doing.
Don't try and impress the world and get a job in just one exercise. Do 10 or 15 different exercises with the simple goal of doing them well, simply, efficiently and cleanly. Suppress your desire to "prove yourself" with every exercise. Instead focus on workflow, mechanics, basics. good arcs, showing proper weight, good breakdowns, strong poses, solid timing. 90% of animation done professionally isn't flashy or impressive- it just needs to be done really solid. No pops, no wobbles in the motion paths, no weak poses. Simple done well is far more valuable than fancy or 'impressive' done sloppily. Like anything you need to focus on developing your strength with the basic things first. So don't even think about a demo reel yet. You need to know how to animate before you can worry about a demo reel.
Animating is fun for me. Sadly for many guys today in animation, they don't love animation as much as having a cool job or a fat paycheck. I would animate even if I never got a job animating. That's how you need to approach your work right now. Worry about the job later. Do your work now with a strong emphasis on doing simple things really, really well and trust me- the job will take care of itself later.
I've been lucky enough to learn and discuss the craft of animation from some of the best of the bests like Keith Lango, Jason Ryan, Victor and a few other super talented guys. And something which I've noticed in common... all these great animators work hard and they never give up easily. They have a simplicity which inspires me the most. They take a super simple idea (could be technically complex) and simply break it down in such a way that its no longer complex in any way. They composite the idea together and the end result is totally jaw dropping. When I talk to them or learn from them, I not only observe their technical approach towards animation, I also pay attention to their mental approach. I'm just trying to follow their footsteps.
I just want you to get into that kinda approach, where you not only enjoy the end result or the final render but also, you enjoy the EVERY single process of animation....from story boards, to 2D planning and animatics to final CG render.
Aaaaaaaaaaarrrrgggghhhh..... I've made this post an essay! Never mind.. I've a bad habit of doing that!
Again, you have a lot of scope for improvement and you're doing well, so keep working. I hope this helps. Good luck.