Aqualung - a classic. But you've probably heard most of it ("Cross-Eyed Mary", "Locomotive Breath", title track, etc.).
Minstrel in the Gallery - completely underrated (althought obviously not on this board -- good job guys!). Lots of acoustic guitar work, but not completely lacking in ...ahem... kahunas.
Thick as a Brick - great two-part concept album. David -- from what I know of your tastes, this would be my recommendation for your next step.
A Passion Play - I really like this one, but I think anyone recommending it as an introduction to Tull hasn't listened to it lately. It is a bit weird. Definitely recommended, just not first.
Songs from the Wood - another classic! Has some of my favourite lyrics in "Hunting Girl" (alright, now you all know that I'm a pervert, but what I really like is the clever wordplay).
Heavy Horses - recommended if you like Songs from the Wood.
Broadsword & the Beast - surprisingly good, and completely underrated, this is Tull at his (sic) most accessible. IMHO. Also, love the lyrics to "*****willow" (more clever wordplay): "...down-fur-lined avenue..." etc.
A - just wanted to mention this one because Eddie Jobson does an appearance on it, and I really like anything he touches. One of their least popular albums, for some reason, just goes to show that at their worst they are still quite good.
Stormwatch - Recommended if you like Broadsword and the Beast.