1. Currently there are too many games on steam and the worst part is that more than a half are crap games made in 1 hour and sold between 0.30 and 5 dollars. Steam shouldn't allow all kind of games, there should be a quality check or those should be moderated by special delegated people.
2. Achievements should matter. All achievements must be protected against software like SAM that unlocks them in a matter of seconds. Also, steam shouldn't allow games to have broken achievements and more important they should be possible to be unlocked even by non-hardcore gamers.
3. If achievements were taken seriously it would be cool some kind of reward for them, perfecting a game should reward the player with game related items (trading cards, emotes, backgrounds etc) and Steam should have badges for total achievement count milestones (100, 500, 1000 etc). Also, desktop backgrounds should be considered as a perfected game reward (something close to the existing system in Uplay).
4. An UI rework is a must. The interface needs to be more similar to the big picture mode which looks quite nice. One awesome feature would be a wider variety of themes (different background styles and colors) similar to what Warframe added in the latest big update called The Sacrifice.
5. Definitely we need more options to configure the client. After the last updates steam schedules game updates by default and there's no option to disable that.