I understand your concerns.
I had a fairly stoic player running a monk in my last campaign, and he was playing it pretty well. But because of his nature, it was hard for me to guage if he was enjoying himself. Since he didn't want the material goodies that the rest of the party was mopping up, getting him appropriate rewards beyond XP was tough.
Then, in the middle of an archer ambush, he waded through the dealy projectiles, deflecting as he went... By getting into the archers, he freed up the rest of the party to attack. He was grinning ear to ear by the end of the evening- he had basically saved the party with just a few key rolls.
Similarly, when the Druid finally got his chance to Call Lightning, he was overjoyed. Ditto the Paladin facing a Necromancer at close range, etc.
So, while you don't need to generate plot hooks for each PC, setting up the occasional encounter that plays into their PC's core strengths gives each player a chance to shine. And a single moment like that can carry a player for months.
But if you really want to get some kind of hook into your players, one of the best ways is the old standby- the Master Villain, someone that they can fight repeatedly, foiling his minions and machinations time and time again. Just make sure not to use him too often (he can't be the sole source of opposition in the campaign) and you MUST eventually give the PCs a chance to bring him down for good.