Hi again pepo,
This is a much bigger question than you know and my carpal won't let me write a book about it, which is what it would take. So here are a couple quick thoughts...
1st of all I think every HONEST affiliate deserves a chance, but merchants for various reasons sometimes need to be more selective.
2ndly realize that an affiliate may have a couple sites listed with the network, but may have a site perfect for your niche, that they forgot to list. Also a newbie with just a banner farm (your example #1) could possible grow to be a good affiliate. Also some affiliates will sign up with whatever site, but after they test to be sure you convert, could end up building an entire site just for you.
2. coupon sites - many are super strong affiliates, some are shady. So I would just carefully look to be sure they are on the up & up.
3. rebate sites - this is the type of site you need to look at very closely. Some rebate sites are considered "parasites" and steal commission from your honest affiliates. The thing to be on the lookout for is any type of download application. Like a shopping rewards toolbar, that notifies consumers of sales or rebates.
Affiliate fraud is a big issue too, so read this thread and be aware of it. There are affiliates that will join pretending to be shopping.com, bizrate or even 5staraffiliateprograms.com. They'll use the real company name and address but are typically fraudulent affiliates from China. Here is a bunch of info about screening for fraud apps. Be sure to see the latest article I just posted on page 3.
Fighting Affiliate Fraud - Put Your Detective Hat On
There are lots of great, honest affiliates out there so just need to
try to allow the right ones in and screen the bad out.