Can I counter a land-creature?
The short answer is that Dryad Arbor can't be countered because it can only be played as a land, not a spell. Here's the supporting statement from the card's Oracle rulings:


  Dryad Arbor is played as a land. It doesn't use the stack, it's not a
  spell, it can't be responded to, it has no mana cost, and it counts as
  your land play for the turn.


And the card's reminder text:


  (Dryad Arbor isn't a spell, it's affected by summoning sickness, and
  it has "{T}: Add {G} to your mana pool.")




If you prefer rules over rulings, let's start with the rule saying you can't counter land:


  305.1. ... Playing a land is a special action; it doesn't use the stack (see rule 115). Rather, the player simply puts the land onto the
  battlefield. Since the land doesn't go on the stack, it is never a
  spell, and players can't respond to it with instants or activated
  abilities.


Moreover, you'll want to note this section of the comp rules:


  305.9. If an object is both a land and another card type, it can be played only as a land. It can't be cast as a spell.


Essentially, this rule is saying that the Land card type overrides the normal behavior of the various spell card types.

This rather esoteric rule isn't actually required for Dryad Arbor to work the way that it does -- simply having no listed mana cost would mean that you can't play it like a normal spell, while being of the Land type would allow it to be played as a land. My best guess as to why it's there is that the original multi-typed lands, Mirrodin's artifact lands, were much more "land" than "spell," so this was codified in the rules for playing them. I take it as not so much saying that a land with a mana cost would still be uncastable as precluding the existence of such a card altogether.