I don't wear pants, unless they're PVC, which means they're usually skin tight.
I usually design my own stuff and friends make them. Except for the exceptional corsets I own from London.
But were I to shop randomly these would be important:

One off's- I hate mass production of anything, unless it's non-descript like PVC pants for instance, or your average Mortisha velvet skirts- there are plenty of those afloat but everyone makes them individual in the manner they wear them. But this is not cost effective for a lot of people which is sad.

Don't steal designs if a client brings one of their own.

Excellent finishes- i.e. the inside musn't look like a frenzied attack by a rampant sewing machine! It's imperative the item looks as good inside as out- hence why I love it when local designers put purple satin lining in corsetry for instance!

Get sizing right, that is crucial- variety of sizes too! Some stuff from Black Rose I tried on had the most obscure sizing- bust 38 just about, waist 32 I'd say and arms and shoulders somewhere in between. Which makes the item good for individual parts only- cutting up the bottom to become a skirt, sleeves to be worked onto another top etc. So proportion with sizing is vital.

I cannot give you examples of actual styles because I think starting out these issues are really more important. When it comes to design, don't copy, go with your gut, your inspiration. Be creative. Look around you at what people are wearing and then transform it into your individual style. Be inventive, that is what will get you noticed!