Ruy Lopez, why is it so effective?

Something I've been wondering especially since I've had a bad time usually playing aginst it is why is the Ruy Lopez so strong? What are some typical problems that Black faces (besides the double pawns in the exchange varation) or mistakes that Black has and makes? 

I still know my biggest problem is my middle game especially looking at some GM Ruy Lopez games 

I would consider looking at some youtube videos before asking about openings. It would be quicker and easier for you to do than wait for someone elses statement who doesn't understand it as well as a master. But I will admit, multiple ideas from different people can definitely help you understand what moves, not to play.

Here's a video about the Ruy Lopez with some explanation by an NM.

https://www.youtube.com/watch?v=_Bs7UtQR58Y

Also, this should go into the Chess Openings Forums.

Tension.  

Writer William Winter said it is an opening for masters, indicating that it is too complex for lower strength players.

Yeah, masters say lots of silly things about chess and learning chess.

Thanks for the great video link, Fenris. Although I'd memorized lines of the Ruy Lopez before, I never fully understood why I was playing the moves I was playing, like avoidance of the fork trick, the timing to exploit lack of ...d6, etc.

Here's the reason Ruben Fine says the Ruy Lopez is so effective:

----------

(p. 16)
      Ruy Lopez
1 e4 e5 2 Nf3 Nc6 3 Bb5

Since White's only trump is the
initiative or the extra move, he is
the first who can attack anything.
Consequently, to force the game
into favorable channels he must
use threats. 2 Nf3 fits in, because
it menaces the e-pawn. And 3 Bb5
is the most logical continuation
because it attacks the defender of
the e-pawn and thus continues the
series of threats. It is no surprise
that of all the openings in this
chapter the Ruy Lopez is hardest
for Black to meet, and understand-
ably the one encountered most often.

6

   One reason why the Ruy Lopez
is so strong is that the most natural
sequence of moves leads to an
ideal position for White. Thus: 3
... d6 4 d4 Bd7 5 Nc3 Nf6 6
O-O Be7 7 Re1 exd4 (forced) 8
Nxd4 O-O 9 Bf1! (6) White has
(p. 17)
the better pawn centre and Black's
position is badly cramped.
   It took a while for experts
to appreciate the strength of this
and similar lines. The two main
older defences--which are now
the buffalo of the e-pawn open-
ings--dominated the stage right
up to the twenties, but then gradu-
ally disappeared.

Fine, Reuben. 1989. The Ideas Behind the Chess Openings, Algebraic Edition. New York: Random House, Inc.

Because your opponent is always facing into a blinding sun.

<Chessmicky> +1 !!

The Ruy is really resilient... for both players.  You have to make a lot of mistakes (or 1 really bad one) before your position is much worse.

That + the flexibility of plans + a closed position is what makes it so strategically dense.  An opening for masters?  Even GMs can have trouble.

Get a few model games from e.g. Fischer or Karpov or whoever.  Learn a few of the different ideas.  That will give you a basis to start building some knowledge.