Some schools (ie Davis) are very explicit in that they look at the academic portion of your application in halves: one half GRE, one half GPA. I'm not sure whether strength/difficulty of classes falls under that GPA half - that's a more fuzzy analysis. That info is
http://www.vetmed.ucdavis.edu/StudentPrograms/StudentGuide_Post.cfm
Basically, if your GPA is weak, rock the GRE. Vice versa - if your GRE is weak, you'd better have an amazing GPA. I think this is a pretty fair way of doing it. For myself, my GPA coming from a very well-known, difficult institution and taking hard classes, was a somewhat average 3.5-3.6ish depending how you calculate it. From where I am, that's actually an excellent GPA, but it is lackluster on paper unless you know the school. My GREs made up for it - I only took it once - verbal 690 (96%), math 760 (86%), writing 6.0 (96%). I got in everywhere I applied.
Here's the detail of Davis' evaluation. Of course each school is different.
I. Academic Factors (50-60 percent)
A. College course work (25–30 percent)
1. Overall GPA in undergraduate/graduate course work
2. GPA of required pre-veterinary medical science courses
3. GPA of last two years of undergraduate/graduate work
(minimum of 72 quarter units or 45 semester units)
B. GRE (25–30 percent)
1. General Test (Verbal, Quantitative and Analytical Writing)
II. Nonacademic Factors (40–50 percent)
A. Personal statement/experience (5–20 percent)
B. Letters of evaluation (5–20 percent)
C. Interview (5–20 percent)
III. Veterinary and Animal Experience
Substantial experience with animals is required. The requirement for animal, veterinary and bio-medical science experience is a minimum of 180 hours (4.5 weeks) at the time of application. Admitted students have an average of 2,500 to 3,000 hours, which includes experience working with veterinarians.
Of note- the most touted "3000 hours" stat re: davis includes both animal and veterinary experience, so dogsitting, equestrian competition, kennel-cleaning, etc all count!