Where'd my game go?: This is really more of a gaming media issue than a steam issue. If a game is on steam, its extremely easy to find, every game has a list of similar games next to it to organically feed into other games. The fact is you won't be able to find every game on a generated list of suggestions when there are hundreds of popular games. Other than a brief moment under "new releases" a sales platform is never going to be the primary means of advertisement for a product.
Where'd all these games come from?: Steam has never claimed or tried to be a gaming gatekeeper. In fact it has went through efforts to allow small indie developers to be able to publish on steam. With ease of access means that more shovelware garbage will flood the system. Honestly the best I would even consider is a system similar to Xbox 360 xbox store and xbox arcade. Basically hiding any non-AAA developed game behind an "indie/small dev" filter or checkbox. Keep in mind this will infuriate pretty much every single indie dev as now their products are listed with the rejects on steam instead of the front page.
Why does the Steam sale suck now?: Refunds. Since you can refund games, you can't have 90% off sales, since you don't have short window 90% off sales there isn't incentive to keep checking the store 4 times a day, since there isn't incentive to check the store every 4 hours, you just buy the games you want and don't look back. And since there's no reason for users to keep checking in, there's not really incentive for steam to make mini games and spend money to give them things to do.
So, what should Valve do about it?: They have games queue and games curators for organic discovery, the bloat of crapware is one of the important issues that I think could be alleviated with soft blocks. Basically if there's too many refund or negative reviews for a game, it is removed from anything other than directly searching. If a single developer has this on too many games, any new release are auto banished unless redeemed. Also they do need to hire at least a small team to start removing the worst of the worst offenders, if not already being done. Steam sales aren't going to get better, but they can find better ways to engage players, such as their "Name your own category awards" interaction has to shift with shifting market.