As count of a limited crown authoritative kingdom, I always expand my territory militarily with a Claim Title CB. I did this in France and HRE and it really makes a difference. Eventually you can challenge the duke himself if you've gained enough of the lands of his vassals.
I'm in a very interesting game that began as the count of Capua in southern italy. It took many tries because the Moors getcha, but I eventually got a good game going. It was hard fought, but eventually, through marriages, wars, crusades and assassinations, I managed to get my dynasty into the King business, namely the king of siciliy.
Since that, I've expanded to Galilee through crusades, Spain through holy wars and Ireland through crafty marriages.
All in all, it's been a very fun game.
I haven't started a game as anything but a count yet. It's good to be the underdog.