I'm kind of making an assumption that almost anywhere in Canada, winter is the largest limiting factor affecting deer populations. At least locally, a long, hard winter will kill more deer than hunters, poachers, yotes and cars. As such, I would recommend winter cover/feed being the most important habitat feature.
Here in NS that means dense softwood stands adjacent to areas that have available browse (within reach for the deer).
It's easy to get caught up and try to make your woodlot the "perfect' habitat area, but take a look at the landscape level, the area that a deer will cover as part of it's home range, use aerial photos and look to see what you have lots of in the area and what is lacking. If your in agricultural country, feed shouldn't be an issue for three seasons, you'll need to think about winter feed sources.
If you're in hardwood country do partial harvests, group or single tree cuts, strip cuts work well also. Cutting hardwoods will result in sprouts/suckers that will provide feed for several years along with the slash from the harvest.
As for the 2-4-D, personally I'd avoid it, it only sets the trees back, doesn't normally kill them outright and will kill any herbaceous species on the ground.
YMMV