Felon said:
I don't know if I'd try to preserve every aspect of the WHFRPG when making a d20 conversion (after all, at some point you have to ask yourself what you're trying to gain my doing the conversion). Personally, I relish the adversity of Warhammer, but not the notion that starting characters should be feeble. What fun is it to be powerless when some nasty chaos daemon pops up on the board? Brandish that great weapon and have at it!
I agree. There is a point in the conversion process where you have to ask "Why not just play Warhammer?"
I think that part of the feel to WHFRP is that when you encounter a chaos demon as a "low level" character...
YOU RUN AWAY! 
The grittiness and the lower power level (at least starting out) in WH helps to set the tone, IMHO. Characters have to use their wits more than their swords to get through the day. If they're tough and smart, they survive and get more powerful. A wizard's apprentice isn't necessarily
feeble, he just can't count on spells to solve every challenge. Just like a novice warrior can't fight his way out of every problem.
When I've played WH, part of what kept me from feeling too powerless was the setting itself. The Old World is full of cities and towns. Most of the adventures I was in took place "in civilization" instead of dungeon crawls. Swordfights and blasts of magic in the streets would have caused more trouble than they were worth. Roleplaying, rogue-skills like bribery and the like was more important than combat. Wizards are handy because folks are scared of them (just steer clear of the Witch Hunters!)
One adventure, we had a few messy fights in alleys and a
wicked encounter with beastmen in the hold of a cargo ship. (actually, the wizard was quite useful in the ship fight because his light spell wouldn't set off the gunpowder in the hold). But most of what we did was dodge city guards (we'd been framed by a corrupt merchant-cultist) and look for stolen weapons. It was a blast.
I know that 3e can be made to fit this kind of setting & tone to a degree. The fact is, as characters in D&D progress, they become powerful enough to ignore most mundane challenges. Even lowering the Massive Damage limit isn't much use when the character has a +16 to their Fort Save v. a DC15! That kind of "Oh, I just let the orcs swing away and concentrate on the evil cleric!" doesn't work in WH, and that's part of what I really like about it.