The major advantages of Wildfire is the minimal amount of die rolling involved. You have a roll for initiative after the command markers have been placed, and the opposed rolls in combat.
While I haven't had much of a chance to to play Warhammer, I have read through on older edition (1998) of the 40K rules. Provided what I have still read is accurate, there are quite a few major differences.
<ul></li>At least three different die rolls. First there is a roll to hit, the a second to see how many of the hits inflict a wound, and finally a roll to see how many soldiers shrug off the damage rather than fall.
WH40K units can be routed, and fall back until they hit the table edge and retreat from play. Wildfire units can be rendered less effective, but not destroyed by morale loss.
Wildfire is an effects based system - you determine a weapon/units ability and then describe what it is, rather than having a point cost and list of unique items to pick and choose from
No special abilities like shoot twice if only half moved or less appear in Wildfire
Order of battle and specific army lits are a major part of 40K, but not WF.
Information in Wildfire is of a general "WYSIWYG" format - a unit's firepower rating is a bonus to its attack roll, no need to look up on a chart as to what roll is needed.</ul>
As to the 15mm scale - standard humans move 6 inches (though it can vary) and the medium range of firearms is from 4-7 inches depending on class (pistol at one end and rife at the other, though you're free to go beyond the baselines)
A page by page breakdown works out to:
<ul>Contents and Setup: 10 pages
Infantry Rules: 39 pages
Fire Team design: 6 pages (Its not difficult at all)
Vehicle Rules: 12
Vehicle design: 6
Sample Background: 20</ul>
And even in the infantry rules, several of those are one page summaries or half filled with a graphic, so its not too daunting a read.
Thanks for the feedback, I'll include comparisons in my next review if I get the chance. I hope this helped.