Mixed ultimate can be both a challenging and incredibly rewarding environment for female players. I'm sure the same is true for men, but I'll stick to what I know.
Many women learn to play in a women's program. There are definitely exceptions to this rule, but for the most part we're used to each other's limitations and taking advantage of those limitations. In most cases, when a female thrower enters a Mixed game for the first time, she finds her hucks are more difficult to use and they are often discounted as less effective then her male counterparts'. When a fast female receiver enters the game, she finds that one step deep of her women is no longer enough to ensure a wide open deep threat, and that which used to be her biggest weapon is shut down by poaches. I can tell you that, especially for younger players hoping to develop a well rounded game or to be impact players, that limitations like these can be incredibly frustrating and enough to keep many women away from the Mixed game. However...
Women can be, and on Nationals caliber teams are, pivotal to the success of Mixed teams.
In terms of the ability to be impact players, I see the most significant opportunities for women to be game changers as handlers or defensive players. Strong female handlers in Mixed are valuable for several reasons. First, they are much harder to stop via help defense than women cutting deep. Women handlers are more likely to face a standard one-on-one defensive situation, and therefore use the full range of their offensive cutting skills than their downfield counterparts. Second, women who aren't shy about breaking the mark (or hucking) can quickly eliminate two Mixed defensive strategies:
A. Down-field male defenders have to worry about their man getting the disc on more parts of the field and can't relax on defense as much as when they don't expect a woman to put it deep or attack the break-side.
B. The female handler's defense can't poach into the cutting lanes because the risk of giving the handler space and time with the disk is too great.
C. A good female handler is a valuable asset in zone games if she can play against a male cup and create a gender (hence speed) mismatch that can be attacked elsewhere on the field.
The best example I have seen of the value of a female handler's strength has been the inability of any team to stop Kira Frew during Team Fisher Price's world's appearance (and victory) in Perth in 2006. That was a talented team, but Kira's cutting abilities, speed, and break mark throws played an undeniably central role in their success.
Strong defense is obviously a fundamental trait of elite players in any division, but women can develop big roles on defense in Mixed primarily because a D is a D (and has the same benefit to your team) regardless of which gender executes it. A man on O can't really stop a good woman on D from being effective. But the reverse is certainly true.
Skilled female defenders are obviously critical for person D against teams with potent female cutters, but are also helpful when you face a team that's not as good at using their women on offense. In this case (and sometimes even against strong female cutters), women can play switching, sandwich-type defense on the open side and look to help the men on D.
A valuable role for a good marker, especially if she's also fast, is in the cup in zone D. This is an interesting strategy for the same reason as getting a woman involved with a zone offensive: a female cup playing against male handlers can create gender mis-matches downfield, making it harder to get the disc to the poppers or deeps. I've had the good fortune of playing with women like Hannah Griego who are capable of generating blocks on male handlers in zone—talk about an effective strategy to undermine your opponent's offensive confidence.
Obviously there are plenty of examples of effective women in other roles. Whore$hack's lady cutters were always really tough, Brass Monkey's Jen Mader and Heather Brown quite possibly led us in goal scoring, and from what I saw of Shazaam 2007 their women were difficult to stop everywhere. I definitely respect those teams (and women) for figuring out how to use their strengths across the board.
Finally, a few specific pieces of advice for Mixed teams with respect to using your women:
1. Practice using the strengths of all of your personnel. In particular, if you have women who can huck then you should practice the timing of pull plays that use their range. If you have fast female receivers who will almost always out-match their defense, practice keeping your men out of the deep space.
2. Design stopped-disc and endzone plays that utilize your women's strengths (as well as your men; any good team should use your entire squad to its potential).
3. There are several good reasons to go four women on O—you feel better about your female match-ups, you want to reduce the number of men on the field for help D, or you expect a zone and want to take a man out of their zone set.
4. In the same vein, develop a zone defense based on both 3 and 4 women on the field so you can throw it if your strategy requires, regardless of what the O calls.
5. Have a Mixed gender strategy committee on your team to make sure the female perspective is heard (both for season-level philosophy as well as game-time adjustments).
6. Prepare to have a dynamic offense, but don't drastically change how you use your women in a tight game due to panic. (I've been in games where this happened, and it is neither fun nor effective).
You don't have to do these things to win games, but teams that do these things right have winning seasons and provide an increasingly attractive environment for elite female players to play.
After winning too many college championships to count as a player and captain with Stanford's Superfly, Lauren turned around and coached an underdog Stanford team to a surprise Championship of their own in '07. Lauren currently plays for San Francisco's Fury (representing the USA at Worlds this August), but was a top handler in the Mixed division, playing with 6 Trained Monkeys in Massachusetts and Brass Monkey in the Bay Area, as well as Huck Finn (2-time Paganello Beach Champions).