Depending on your level of technical facility and the group you're being judged with, judges will look for different levels of interpretational depth. The first thing that will get you kicked out is inaccuracies; practise with a metrenome every day with your hands seperate and then together, working each up to the marked tempo and faster so that you have complete mastery and confidence on each passage. The next level of achievement is tempo- are your tempos close to those marked, and do they allow you to properly express each phrase?
Next is phrasing, which is the art of making each line of notes into a musical idea. This has to do with changing dynamics and the use of rubato. There are many ways in which you can shape a phrase;
-Directionally- crescendo to the top notes of a phrase or, conversely, if the line moves downward, to the bottom notes.
-Rhythmically- crescendo to points where many shorter notes are followed by a longer one.
-Harmonically- crensendo to the point of greatest dissonance and decrescendo where the dissonance resolves.
-There are a few others that are used in more specific circumstances; if you continue to train your ear, you may find that these others have obvious places in certain spots.
Getting deeper into the realms of specifics, judges will look for good voicing. When playing material where there is more than one note in each hand, the melody must be heard above all else. In contrapuntal music such as that written by Bach, Beethoven, Mozart, Shostakovich and Rachmaninoff, beware that the melody may not always be in the right hand, or that there may be more than one melody going at the same time. Look for notes whose stems point the opposite direction of those around it; this often denotes melody notes in a body of harmony. See the Intermezzo in Schumann's 'Faschingsschwank aus Wien' for a good example of how to voice things well.
Now, at the deepest level of performance technique, we come to artistry. This category can be divided into two sections; how well the piece is played according to the standards of the period in which it was written, and how well your interpretation conveys the mood and spirit of the piece. In the first section; pieces by Bach or Telmann are not to be played with extreme changes of tempo or sudden dynamic changes, especially pieces intended for more than one musician, such as a concerto. While a Bach well-tempered Klavier piece can have some rubato, a concerto with orchestra should be strictly in tempo expect for the final cadence. Music of later composers such as Rachmaninoff, Schumann, Grieg, Dvořák or Liszt, however, are intended to be much more liberal and rubato is therefore at the performer's discretion to a certain extent.
As for personal expression, the judges will be looking for the quality that you bring to the piece through your interpretation. The biggest mistake you speak of is most definately the lack of expression that some people manifest. When you've practised for so long that you know the piece well enough to look around the room while you're playing, this tells the judges that you're not interested in what you're doing, that you're not involved. A good performance would be spellbinding in its involvement; the performer looks confident, sits at the piano for a moment before lifting his hand to the keys, perhaps gives a downbeat with his body to show that he's been considering the tempo before starting. Then the performer stays focused and passionate throughout the entire piece, never looking like what he's doing is boring routine to him. If you can convince the judges that you enjoy what you're doing and are motivated and involved (while still being accurate and precise), you will have beaten out the vast majority of your competition already.