How do you know if a game was "worth playing"?

I know the standards I use to judge when a game is worth it's cost in time and money (I focus more on money because I'm a broke grad student), but I wanted to get opinions from the rest of the community on this one.  I enjoy writing reviews, but I've been stumped by finding a respectable measure of "worth playing".

I've been trying to figure this one out, and the standard I've used was $5/hr.  If a game entertains me for a day (2-3 hours), but was only $5 or less, then it was alright.  That's still favorably comparable to a movie.  What are the standards that the rest of you guys use?  Do you even consider the cost of a game when you're looking at it?  It's usually pretty clear to me that it's not worth $50 or more for me to buy a game that I play for an hour and then hate, but I'm trying to figure out where to draw the line on cost.

The other big issue I run into is "the value of playing time".  I know a game like WoW (or any other MMO) can have hours upon hours of content,  but much of it requires a tedious grind to get to.  Grinding is fun for about the first 1,000 hours you do it, across every game, your entire life.  I don't count any time spent grinding that I didn't enjoy as valuable gameplay (so if I have to grind for 10 hours before I really get to play, then those hours don't count as real content when I'm judging the game).  How do other people measure the useful hours in a game ("useful hours in a game" is such an oxymoron).