Features - Definitely. Whenever I buy any gear for biking, snowboarding, etc, I start the buying process with certain features in mind. The list may vary in length depending on what I'm buying, but if an item doesn't have all of those particular features then I'll just keep searching.
Size - Yes. The range of sizes that hydration packs comes in is massive, so I'd looking for a pack that is a particular size.
Comfort - Yes. I was recently looking at some MEC packs recently, and I didn't get one simply because they just didn't fit me very well. Something about the straps just didn't seem right.
Brand Name - Somewhat important. As a name, Camelbak means very little to me (other than 'overpriced'). Dakine means A LOT to me - I wish I bought stock in Dakine a few years ago because of how much of their gear I buy. My present pack is a Hydrapak, and I'd potentially get another Hydrapak as I've been happy with my present pack. MEC - I checked their packs out because they're usally functional and a good value. I think the name itself might mean more in terms of pedigree/reputation than name-brand-coolness
Price - Yes. I am both somewhat cheap but willing to spend money for quality items if I feel like it is a good value. I doubt I'd ever get a Camelbak pack as I view them as being over-priced in my mind (people with good memories may remember similar rants about Burton from this past winter).
Cool-Factor - What is cool to me is stuff that works - you can't become cool through advertising, you have to earn it by making quality products. Although I am willing to admit that I am probably susceptible to advertising so it probably is a factor to some extent. I realized a long time ago that what a 15 year old views as cool and what I view as cool probably aren't very similar.
Friend's Recommendation - Counts for a fair amount, but I've learned over the years to take it with a grain of salt and to make sure that any pros or cons would be for comparable circumstances. If you're raving about the fit and durability of a really small hydration pack you use for xc, and i'm looking for a bigger pack that can handle the abuse of freeriding, your recommendation isn't overly useful.
Pro Endorsement - I couldn't even tell you a single pro with a pack endorsement, and even if I could it would mean very little to me. It's not like Mr Pro Biker is actually winning races or slopestyles or whatever with the pack on.
Advertising - I don't read mt bike magazines, and am on this site so often that I've grown immune to ads (maybe…must go open NSCU bank account…).
Pedigree/Reputation - If this is referring to a company that has a long history of making quality products, than yes - especially if I've had a product from that company before that I've been happy with. Once I've used a companies product, if I've been happy with it I'll stick with that company for a long while.
Community Involvement - Maybe as a tie-breaker for making a decision. It might make me more aware of a brand that I wouldn't otherwise know about, so it might be useful after all.
The only hydration pack that I have ever used in a Hydrapack. I bought it a few years ago before I knew much about hydration packs, and am lucky that it has worked out as well as it has. I'm presently looking for a slightly larger pack, and am leaning towards getting a Dakine Nomad. Before I buy anything, I'll check out several shops to see what all of my options are, focusing mostly on features and price. There, after all that typing the important part comes to the surface.
Sorry to everyone else for the long read