I'm not really thinking about the portal as patting good products on the back. If you have a good product you'll suffer a big drop in sales when it drops off the front page of RPGNow. As RPGNow is so successful with publishers it means that you drop off the front page really quickly.
As an example, Forgotten Heroes: Paladin and Unearthed Adventurers both had similar first month sales, but as Paladin was released at a time before RPGNow exploded it stuck around and generated a further 3 or 4 good months sales. Unearthed Adventurers had the first month's sales and then slowed right down to the same sort of trickle paladin did after 4 months. There may be something in the relative appeal/quality of the products, but I'm sure that the time spent on the front page at RPGNow plays a big role.
Therefore a portal site with a more restrictive mandate and an assurance of quality, I believe, would be well received by consumers. It would help sales by providing a better reliable quality assurance and retaining a shelf of new products for 2-3 months rather than 2-3 weeks.
I'm not suggesting alienating non d20 pubishers, I think two portal sites, one for D20 and one for non D20 would be ideal. Maybe you could have another for Wargames and another for support products, such as card models and clip art. At the moment I think we all interfere with each other's sales by pushing so much through the front page.
Cheerio,
Ben