So: Step One: What type of personality is your character going to have?
Every point you put into a stat or a skill, and every feat you choose does make a difference. However, the game does forgive you for making mistakes here and there - because after all, every TWO ranks you place in a stat only ups your overall bonus by ONE. This means, if you didn't horde a lot of points in a stat, you are only down by maybe one bonus point in the game.
So, indeed, it is how you play your character's strengths and weaknesses that is the definitive factor of character creation.
Every skill and feat and stat have points and counter points. The Knockdown Feat has the Discipline skill-counter, for example. There are a lot of people that employ Knockdown, and Monks recieve the Feat for free after so many levels. The Discipline skill, however, is only available to the Paladin, the Fighter, and the Barbarian. Of monsters in the field, you can basically use this 'fighter/barbarian' template to judge if a monster can resist your knockdown well. BUT, how does knockdown alter how a character is roleplayed? Isn't this, after all, about personality?
I think as a roleplay device, knockdown has two things - the good warrior who doesn't want to run into anybody - he knows his ability to knock someone off their feet if he isnt careful, and the bully that doesn't mind pushing people around.
So, if you want to get into the mechanics of the game, and make those mechanisms something to enhance your RP of your character - using the numbers can help steer you in a direction.
Step Two: What are my options?
Positions on the Field: The options are laid out before you. There are a few combat strategies to employ - Frontline Fighter, Mid-range, Ranged Attack, Backdoor Defense. Any character can attempt to take one of these positions and be employed strategically. A scout may hide in shadows/drink invis and move into the Frontline, hoping he doesnt get noticed. But Mid-Range is for the Medic. If you want to heal your comrades in arms, you need to be in a slot just behind them, ready to take up arms if they fall. Even a Barbarian can take this spot, using healing kits before moving forward into the frontline. Ranged Attack is best for Archers and Mages. Lastly, gaurd your six. (farthest loc from enemy)
Classes: Rogues, Clerics, Fighters, Wizards - these you have to study on your own. However, you should have an idea of what you want to play before you look into each one. These classes determing your base abilities, and is actually the key-factor in determining if your character is successful or not. They lead your stats, feats, and skills in the direction that the class can go into. Therefore, one needs to really get to know them to know which to pick and why.
I will use a Barbarian here again. The Barbarian looks like a Fighter with more hit points on first glance. Don't let your eyes fool you, there are some big differences between a Barbarian and a Fighter. A Barbarian gets 4+ int mod for skills, allowing him to Taunt his enemy more effectively than a Fighter ever could, and the Barbarian needs a high dexterity to keep a decent armor class unless you either take a precious feat in Heavy Armor, or dual class him. A Fighter doesn't get a whole lot of skills to use, but gets feat after feat, allowing a character to modify a fighter quite a lot in what he can do in the game.
For instance: Fighter Level One :: Human
I want my Fighter not to worry about Discipline as a skill choice. So, I give my Fighter a few more points in Constitution in case he gets Knocked down, Then give him some average points in Wisdom. To counter these choices, I take away from his dexterity, and his intelligence. I take Improved Unarmed Strike, Knockdown, and for giggles, Weapon Focus: Greatsword. Then later on, when my Base Attack is high enough, I get to pick Stunning Blow. Now if someone wants to disarm me, I have a perfect counter - I can knock him down, or stun him, get my weapon back, and keep fighting. Stun takes the counter skill of Concentration to resist it, and Knock-down takes Discipline - by having two ways to attack my opponents stats, I open up my options.
With a Barbarian, I would have a tough time getting the feats together to try that tactic. I would simply use my higher skill points to resist the Knockdown/Stunning blow, and hope my rolls are good!
This is all very well and good against foes with this feat, but the foes usually throw a lot more at you then just a knockdown. Hence my firm belief that diversity overcomes adversity in AD&D.
This is where you come in. Yes, you! Your character could be flawed in your eyes and different in action than what you saw on paper when you made him or her. That may not be a bad thing. A character can really come into his or her own within the confines set by your class. Look at what you CAN do instead of focusing on what you CANNOT do. Keep your strengths in focus, and if there is trouble - we all get by with a little help from our friends.

Good luck out there, and I hope this post was useful to you.