How can I improve my deck building skills?
It does seem, from your description of your play style, that you do have a problem in consistently misinterpreting what things help people win Magic games.  Let's have a look at some of the things you say you like, and that you underestimate:

LIKES


Lifegain - with a few rare exceptions, lifegain cards are TERRIBLE - they do nothing to help you win the game, just slow down the rate at which you lose it.
Creature buffs - if you spend a card putting +1/+1 counters on a creature, and your opponent Terrors it, you've just 2-for-1'd yourself.  Not good.
Creature destruction - okay, your instincts are correct on this one.  Removal is and always will be king.
Mid-sized creatures - these tend to be the least efficient choice.  Either you want highly efficient "weenies" who can win the game for you fast, or you want massive monsters who can end the game in a couple of swings.
Enchantments of all kinds - I assume you include Auras in this.  See the 2-for-1 problem I mentioned under Creature Buffs, above.
Mana generation - it would be fair to say that mana ramp strategies can play a part in successful Tier One decks; but you need a definite game plan and a strong likelihood of consistently using all your mana every turn.  Just having more mana than your opponent, while he's swarming you with creatures, seems like a really bad place to be!


UNDERESTIMATES


Direct damage - this is obviously amazing, because it can act as 1-for-1 removal early on to keep you in the game; but then go straight to the opponent's dome later, when you're only a few life points away from victory.  Look at Fireball and it shouldn't be hard to understand why this is a top pick in almost any draft format.
Creature control - Mind Control is also a #1 draft pick.  By casting it you not only gain a creature, you also steal the opponent's best creature, making it a sort of automatic 2-for-1.
Artifacts - well, these vary from good to bad, but one of the great things about artifacts is that they don't require a complicated manabase to pilot.  Artifact decks never get mana screwed, so you shouldn't underestimate them!
Card draw - something like Divination may not look impressive, but you need to get your head around the idea that getting 2 cards return for an outlay of 1 card is the cornerstone of winning games of Magic...


It seems to me that you would do well to learn more about the principle of card advantage.  If your opponent plays a card and you remove it with a card, you're level pegging.  If you play two creature cards and he removes them with one card, he's a card up.  If you play a card that gives you 7 life, he's also a card up.  Whoever is the most cards up at the end of the game often wins!  The alternative, of course, is blazing speed: kill your opponent before he can get too many cards up on you...

I could talk about this stuff forever but this answer is already getting long; hopefully there are plenty of things for you to think about here already!  Feel free to ask about anything that is unclear in the comments though.