Mechanicaly, it isn't as intuitive as the later versions of D&D, which aimed at a unified approach (d20 roll + modifiers = DC or higher to succeed). There are situations where rolling higher is better (attack rolls, saving throws) and others where the opposite is true (non-weapon proficiencies checks, ability checks). You will see many mechanics that may initially confuse you (such as how saving throws are categorized, THAC0) but then you'll get them (THAC0 = an inverted BAB), or simply learn to live with them (ST).
There are also a lot of things that seem as poorly justified, such as demihuman level limits ("because human are more numerous"

) and multiclass combinations (or dual-class for humans).
In regards to AD&D 1e, it's pretty much what 3.5 was to 3.0, an update which incorporates many changes that were made during the 1e years, so IMHO, the only thing you're not getting is the Gygax prose.
Having said that, the mechanics I fell (or seem to remember) are less thorough than those of 3.x, so it leaves room for a lot of house ruling, as well as less need for miniatures and a battle mat.
Where AD&D 2e really shines, though, is in the proliferation of campaign settings published for it (which, ultimately, contributed to TSR's demise): Forgotten Realms, Greyhawk, Dark Sun, Ravenloft, Planescape, Birthright, Spelljammer, Dragonlance, Mystara... A whole lot to chose from.
