The problem is that there is no way to validate Steam keys without redeeming...

It's not a flaw with gifting really. Gifting implies the thing is going to someone to give, not to trade. Verification is not necessary for that. They have a trade system for trading (within Steam).

Granted, but don't forget someone could be a dick, and gift you something you really don't want (something cheap), and then it gets stuck on your game list. I know, I know, giftee beware, but still.

True enough I guess :)

Yes, but it means that if you post a code, somebody has to redeem it in order to know what it is. So, unless they already own it, they have it whether they want it or not.

Let's call them scummers.

If your friends cause you that much concern then politely decline and tell them to send it to you via Email, that way you get to see what game it is before you activate it :)

Also, if you activate a key, don't you get a screen saying "you're about to activate game X" before you accept? I never had to say no but I figured it was possible?

Isn't possible to decline, you can only choose not to download....
Also, your silly friends can lie to you... ;p

Nope
Steam only tells you what the key is for once it's activate, or if you already have the game.

Then tell them to send an email Gift ^^

Yeah, but you can't do that with a Steam key.... (as far as I know...)

If that's the only thing people can gift you then I imagine not, no. You'll have to judge whoever it is that's offering you I imagine :)

True. It could never 100% eradicate scamming. But it would be a place then to show who said scammer was. Anyone wanting to trade/gift could check the history of the person they are trading with. If the person has no trade history (i.e., new trader/alt account of scammer) then you know to be very very careful.

I've offered to do that but there didn't seem to be a great deal of interest in it.