Keywords in a domain are still fairly weighty for non-competitive keywords, in that it's easier to rank them than branded names. But as far as the long term and keywords with any degree of competition, the effect quickly becomes unnoticeable, or at least easy to overcome.
I would anticipate a minimum 10% loss of traffic with a perfect migration, which you could recover over time. Any migration problems and that percentage can rise pretty rapidly. The most important mitigating factor is how many URLs involved.
Interestingly, at the time of switchover, you will often see an instant traffic rise, as Google discovers the new domain, but has not yet replaced the old one. This effect disappears quickly ;)
The real question here is why you are redirecting. If it's an inevitability (e.g. you've changed brand name) then you can bite the bullet and go ahead knowing that a correct implementation is likely to be relatively hassle-free. If it's because you think a new name is likely to improve rankings in and of itself, I think you're likely to be disappointed.