Looking at the vendors who are most successful (those that appear in the top vendor categories consistently), they seem to have a few things in common: a big back catalog, an extremely fast paced production cycle with very regular releases (at least weekly, sometimes more frequent), outstanding promo art and good (if obvious) marketing text, and frequent sales.

I don't think it matters what you make, as long as it meets quality standards and isn't too obviously niche. What seems to matter is how often you can release something. You need to stay on the front page, with either a sale, a special offer, a new release, or someone else releasing an add-on. 

No doubt this leads to a lot of rehashing of old styles and a lot of products that are very similar to already existing items. It can't be easy to keep up that kind of pace.

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.