1. Melodies are without a direction. Always think where the culmination must be (in small details and in a broad general sense).
2. Rhythms are too unified or too contrasting. In either case, your listener will feel bored. Having a clear sense of meter helps.
3. Harmonies that you don't understand. You should be able to name each chord you are playing.
4. Dynamics are too soft or too loud for a long time. Be mindful of your listeners - nobody likes the same dynamic level over extended periods of time.
5. Registration - no balance between the parts. Try to register your piece so that more prominent thematic material would be just a little louder than the rest of the parts.
6. Texture is too similar or too varied. If the piece is very short, unified texture is a good thing but when you play longer than 2 minutes - try to vary the texture (but not too much).
7. Form is not balanced or undecided. The hard part for an improviser is remembering the themes (which is the easiest thing with written composition) and repeating some of them in a balanced way. Don't start to play until you have figured out what the form should be (simple is OK but hectic is not).
Consider the above 7 things to avoid which will make each of the musical elements more interesting and artistically pleasing. This in turn will help you create a highly balanced improvisation which you and your listeners will enjoy.