There's shooting Hill style and there are Hill style bows. John hit it pretty good on shooting Hill style. That was how Howard shot and it surely did work for him. However, doesn't mean that style will work for everyone. Depends a bit on your physical characteristics and reflexes and what you're comfortable with. Darn good way to shoot, though, and well worth working with... especially the split vision aiming, which, for most folks, can quickly turn into as close to instinctive as exists. If it doesn't work for you, you can still shoot Hill style bows very well, you just find what does work for you.
Hill style bows are true American Longbows (as differentiated from English Longbows). They can have straight limbs, moderately reflexed limbs or moderately deflexed limbs when unstrung. Never, never, never, however, can they have limbs that flex first one way, then another when unstrung. If they do that, they are R/D bows, i.e. "reflexes in drag". Strung, true Hills are a perfect "D" shapre. The rule books came up with a way to classify R/D bows as longbows, using the strung shape definition, and I think that's a darn shame, because they are wonderful bows in their own right and should not need to ride on the reputation of another style or be moved into its class, but that's my personal soapbox. As made today, I feel the average R/D bow will outperform the average longbow or recurve. The Hill style is a true traditional bow and should be appreciated as such and shot the way it was shot in its day.
I shoot both styles and like them, but I'll freely admit that the true Hill style is a greater challenge to learn to shoot well. That's why I concentrate on them. I like the challenge. I also like the simple design and purity of it, as do many others, which is why there always seems to be interest in it.
The fact that we have Hill style, R/D and recurve, as well as selfbows and laminates, traditional building and modern development is what makes archery such a fantastic activity.
If you want to pursue Hills further, visit the Hill Archery web site and do some googling for other references. Drop in on the Howard Hill Longbowmen web site, too, or participate in their email list for awhile. There's a ton of material and sources out there that can help you decide whether you want to go for it.
(Whoa! Where'd all that come from?) Simple answer to your question: Hills are straight and skinny. Everything else is fatter and has wiggle limbs. 8^)