I was a paper boy once, for 5 years. I first spent my tiny salary in heavy, black, death and funeral doom metal CD's. Then I discovered Magic The Gathering, so I bought less heavy music and more booster packs. I knew that I could buy single cards separately, and that entering draft tournaments also was a good way of getting good cards, that one could trade if un-needed later (but I was socially awkward so I did not join this cabooga). But opening a new pack and seeing what Rare golden card I would get was magical. Not the least because I always had a chance at the best card in a whole set. This magic (HÆHÆ) is entirely gone in Steam Trading Cards. What they have can be compared to a booster pack for each game. There are 6 or 10 or 15 or so, cards, and that is it. None of them are rare, nor uncommon. They are all common. So what it feels like is opening a booster pack with an X amount of common cards. That is already pretty demotivating, but what makes it even less fun is that you only get half of these cards. The rest don't drop. The rest you can get by trading or buying, and in collector games, the easiest accessible way is always the one that a collector will choose. In effect, you can collect all cards 5 times by spending around 4-10€ (depending on the game) in 10 minutes. The scarcity principle that makes collecting games so fun is non-existent.
There is no magic in it. No excitement.
To compare to Magic The Gathering: the most unique cards can also be bought, but they are expensive as hell. And sooner or later they will be sold out. Steam Cards won't, because every new player among potential millions will generate new cards.
There is no concept around it, only extremely easily accessible content to anyone who wants it. It is like Diablo 3 items: the most fun legendary items didn't seem so magical once you saw hundreds of them listed in Auction House: not only did it remove the scarcity principle by showing that there are so many of them, but also by introducing the feeling that no one wants them.
It is like taking a pill that makes you full, instead of eating a really nice dinner with dessert. Reading a 20 minute long summary of all 5 seasons of Breaking Bad instead of watching it. Problem is you still want it because it is slightly entertaining, but the dumbed down simpleness of it makes it frustrating.