Some things to consider:
Where will you be smoking: you don't want a pipe shape that'll get in your way (eg. if you smoke while stacking things, washing windows, or using revolving doors, don't use a long straight pipe, etc.).
Appearance: A tall billiard, a stove pipe, or a Canadian might look good on a guy with Abe Lincoln's physique, but it would look clownish on a short, stocky guy. A pipe with delicate, slim lines would look out of place if your features are rugged, etc...
Habit: If you like to clench a pipe in your teeth, or need to, get a deeply bent pipe to leverage the weight. If smoking in the den, reading or relaxing, curved pipes are generally more comfortable than straights.
For me, what
Collin said about bowl capacity resonates well. I know, you don't
have to fill a large bowl up to the top; but you'll soon tire of trying to ignite tobaccos deep within a large pipe -- and there are other issues related to mismatches in bowl size and appetite. It's better to start out with a middle depth and diameter pipe,
or smaller, and by your next PAD attack you'll know if you need something different.
Over the years I've discovered that the single most important pipe characteristic predicting my eventual acceptance and continued use of a particular pipe is
how the stem feels in my mouth. If it ain't comfortable there, it ain't gonna see much use. If you're shopping at a B&M, take a ziplock style plastic bag, or a latex rubber glove, and ask the clerk if you can use it as protection while testing the feel.
One last point, much aggravation can be averted by testing
whether a pipe cleaner can pass smoothly from button to bowl. This will help clear some occasional gurgle without removing the stem -- which requires a cool down period. Same applies to cleaning immediately after a smoke. It means avoiding the p-lip, and staying with a fishtail stem.
(Perhaps someone else can tell you about how awkward "system pipes" are. I don't own one, but I heard you can't tip them to dump out ashes. I dunno, maybe someone can help on that one. If true, I think that would be a royal PITA) See member
Searock's
post in this regard. He's experienced and quite well informed.
Just my 2¢...