Steam is growing up. From it's infant days of Counter-Strike 1.6 & Half-Life 2 distribution, we now see an awkward teenage (perhaps collegiate student) storefront that has Assasin's Creed IV: Black Flag sharing a shelf with Starbound.
There is no immediate problem as this "equal opportunity" storefront has fostered the growth of the indie genre. However, looking ahead, we may experience a discourse of interests when using Steam to shop. With the combination of Greenlight (moving full steam ahead at approx. 500 games a year) and already approved publishers pushing out titles, the amount of poor quality products will increase ( it already has- http://store.steampowered.com/app/226700/) at an alarming rate. Though the institution of readily available reviews and scores will help self-promotion, nothing will prevent the weird wilderness of Steam Daily Sales, Promoted/Featured Games & Top Seller list to be populated with an unhealthy mix of good and bad games.
This is not to say that only good games should be featuredt. This is, however, to point to the Steam storefont's "windows" and ask what credentials will products be promoted by? In consumer packaged goods, sales teams are charged with maintaining relationships with retailers to negotiate and ensure distribution deals, promotions & shelf space are guaranteed. Already, due to the mass quanities of releases, we see games falling off the Steam grid way too fast and essentially be put to retail death. Will indies need sales reps in the coming years? Or will Valve continue to use their "secret sauce" to decide who rises the the top and how long will it be acceptable?
A simple fact remains: it's going to get complicated. More good games will come, more bad games will come & the in-betweeners will fill the uncomfortable gap. Perhaps we should see a reinvention of the Steam store, as Gabe hinted at, that allows a multitiude of "stores" to be created with different hosts and vendors. A Steam marketplace with multiple retailers has the opportunity to further segment the market which is A) dangerous and B) brilliant. When shopping for a crock pot, one can pick up the cheapest option at Wal-Mart, or peruse "artisnal" solutions at William-Sonoma. Imagine the same theory when shopping for a RTS game then go to the current Steam and try to sift through all the Strategy game choices. It's going to get real weird, real soon.