I've identified 5 different types of FF jackasses. Feel free to add more or share your experiences with a particular brand of jackass.
1. Rapist Jackass -- This is the guy that will always try to blatantly screw you when proposing trades. We have a couple of RJA's in my league right now. They'll propose trades of like Kevin Jones and Farve for LJ, Vanderjack and Vince Young for Chester Taylor, or Buckhalter and Kennison for Tiki, and tell you with a straight face that it's a good deal. RJA's are very annoying, and if your league is overrun with them, there won't be much trading.
2. Trash Talkin Jackass -- These are the funnest of the jackasses. They'll talk smack. They'll make fun of you, your team, and your mother. TTJA's are notorious for 'outing' other league owners who are allegedly having sexual relations with one another. And if you lose to a TTJA, you can bet you're going to hear about it for at least the rest of the season, and probably for close to a year. I'm guilty of being a TTJA.
3. Wafflin Jackass -- A WJA is that guy that can't make a decision. Indecision isn't an just an option for them, it's a way of life. WJA's commonly discuss and agree on trades with other owners in casual situations, yet when it comes time to pull the trigger, they get scared and back out because they can't make up their minds. This is the type of jackass where you can offer LT2 for Brady, and they hesitate saying they need to check their backup qb's bye week first.
4. Dumass Jackass -- My personal favorite. The stereotypical DJA is a horrible gm, and has a bad temper too. If they can blame any of their numerous mistakes on someone else in the league, you can bet your house they will, as they have to protect their huge ego because they have convinced themselves that they're really awesome at FF.
We had a great DJA moment in one league last week. DJA has Culpepper and Carr as his only qbs and both are on byes. So in typical DJA fashion, the DJA waits until Sunday morning to pick up Harrington, who is the only starting qb left in free agency. Of course another owner picked up Harrington on Friday to mess with the DJA, so now the DJA is enraged.

It's the other guys fault that he now doesn't have a qb to start. DJA says the other guy didn't even need Harrington, and he has lost all respect for the other owner and will screw him in FF or anything else, any way he can.

He calls the PDJA (see below) on Sunday to complain, only to hang up on him furiously after being told 'You should be a better gm.'
How do DJA's stay competitive? Either through blind luck, or by consulting with at least half of the other league owners when trying to make a decision. By talking to enough people, they're bound to eventually get a good opinion or two.
5. Pump & Dump Jackass -- This is the guy who will pick up free agents/waiver wire claims he doesn't really need, just to mess with other people who need the players he now has (see Harrington example above, that guy is a PDJA). As long as you're not the victim of the PDJA's antics, it's usually pretty entertaining watching him try to work a trade with his latest aquistion, or straight out taunt the other owner. Invariably, the player he picked up earlier in the week is dumped by Tuesday for someone else. If your league has weak rules, a PDJA can have a huge negative effect on the league's intregrity.