My collection is fairly modest (about 13 pipes as of now), but it's hard to resist certain things. I'm not persnickety about it by and large, but I've gotten to the point where I definitely know what I like and what I don't. I don't have strict criteria, but a pipe purchase usually follows these guidelines, a decalogue of sorts:
1. Less than $200, unless I can get a good deal or have some serious cash saved up (yea, fat chance).
2. No filters. Period. I don't budge on this one.
3. No avant-garde shapes. Goofy looking elephant ears, freehands, and just any pipe that doesn't look like I could put it in a decent pouch gets passed on.
4. I do enjoy cobs and meerschaums, but briar is king for me and make up the bulk of my purchases.
5. Color. I'm a sucker for colorful stems in particular. I don't care if it is vulcanite or acrylic, and plain black stems are fine, but playful coloration can be a good way to sucker me into getting a pipe I otherwise would pass on.
6. No nose-warmers and/or overly short stems. I find them uncomfortable and ugly.
7. I like a pipe with some size to it. It doesn't have to be massive, but I find something with at least "medium" proportions just smokes better. Allows the tobacco to open up and gives me more to smoke!
8. A pipe that can straddle the line between tradition and being unique is a winner in my eyes. I prefer traditional shapes and grain patterns, but I like when a manufacturer or artisan adds a little something that makes it stand out just a tad.
9. My home town has no shortage of crappy vape stores manned by clerks who smell vaguely of cat pee, but nary a proper B&M is to be found. So, when I shop online, I try to do so with a provider with a good reputation and return policy.
10. Finally, there are some brands that I find I have some loyalty to. I've been on a Peterson kick lately, for example. I try to not be a fanboy of any one brand, but hey, if I have more than one pipe by the same brand that smoke well, that makes it more likely I will purchase from that stock.