What are the key strategy differences between Settlers of Catan and Seafarers?
For me the big strategy for seafarers is to realize that most of the time the islands are not worth it.  If it is very convenient to go for an island then do it, but usually I find that the resources I expend could have been better used elsewhere.

(Though if everyone is following this strategy, then the islands are great because of the free space.  Basically you want everyone else to go for the islands but you.)