As stated on the coffee talk section, "it is a matter of taste". There are a few rules though that makes sure that the preparation and making of your espresso was done with care.

Use fresh coffee grind
As soon as the grinded coffee comes in contact with air it starts to go bad. Beliave me, there is a big differance between an espresso made from fresh grind and one made from coffee that has been lying around for an hour or two. Make sure to check that they grind when they make your cup.

Extraction time
From the moment the barista pushes the "start button" on his espresso machine you should have between 25-30 seconds to pull out your wallet. It is much to often that you get served your "espresso" before you even have had time to blink. The barista should usually tamp the coffee grind.

Volume
The perfect espresso needs one more parameter, the volume of coffee. More 3 cl can make the coffee loose some of its "body". Less than 3 cl and you have an caffe ristretto (which of course can be a wonderful, tastefull exprerience depending on the coffee blend and the skill of the barista). When the 25-30 seconds are up, you should have about 3 cl in your cup.

Good stuff    :)
Then there are of course the matter of which coffee brand that was used, but if your espresso looks like the above it was at least done right.
Not so good stuff    :(
This is how a not so good espresso usually looks. There could be many reasons why, here are three. The grind is to coarse (as the opposite of to fine) in which case the barista has to check his/hers grinder. The barista did not tamp the coffee. The coffee beans are to old. The barista needs to throw away his/hers old coffee and order new. All the reasons above can make the extraction time to short (under 25 seconds). If the water flows to quickly through the coffee grind the coffee gets thin and bitter. Check out the "Factors in the perfect cup" by a real professional, David Schomer.