I agree that the skill tree is pretty important.  I'm sure that other builds are viable, but I really like the weapon focus.  With the Crusader I tend to go this way with the skills;

1)Searing Blade

2)Weapon Focus

3)Heavy Weapons

4)Whack

(This is where I start getting to the 'If I make it this far' zone)

5) Either Whack again or Faith

You can honestly get faith right up front.  That skill is a real life saver, and helps hugely.  I just leave it for later because I find that getting weapon focus quick provides the damage boost you need for Bandit Holes 3-4, and gives you a good foundation for tackling the dungeon.  

Whack is a really good skill to get because it gives you the ability to juggle enemies.  If you are facing two assassins, for example, you can whack one and kill the other.  The whack will either A) kill him outright due to the instant crit or B)send him flying away if you can't get that instant crit.

I've experimented with martyrin', and it seems like a really helpful skill.  I think it's overlooked a lot because it's not really an offensive spell, and most of the time you want to focus on killing the enemies.  It gives you a pretty decent boost to your armor and has a chance to awe some of the enemies (which acts sort of like a charm as far as I can tell).

I've never really tried holy fires, but I want to give it a shot.  It seems like it would be best suited to a high AC Crusader who just charges in.  You almost want to get surrounded with Holy Fires.  

Now that I think about it, maybe a martyrin' and Holy Fires combo would be really good.  Looks like I got a new build to try!

I was wondering that myself. Just met (and was wasted by) Satan for the first time yesterday. As a crusader, is there any way to keep whaling on him without being subject to that consarned attack that pushes you backwards? Misjudging how many of those I could take before going for a health potion is the reason I didn't make it. Still happy to have gotten that far at all though!