The seasoned prosecutor.
Although it might seem easy to take the stand and tell your side of the story, the accused must remember that the prosecutor will have the opportunity for questioning. The prosecutor most likely has had a good deal of experience questioning witnesses and will attempt to make the accused look evasive or defensive even if it has nothing to do with the case. If jurors feel the accused is not answering in a straightforward way, they may vote to convict even if other evidence is lacking.
The unseasoned defendant.
No matter how innocent a person may be, testifying at trial is an extremely nerve-wracking situation. Testifying in one's own defense is even more scary. Not only are the consequences of a criminal trial severe, but also an accused is likely testifying for the first time ever, while the prosecutor appears in Court on a daily basis. An accused's lack of sophistication can cause jurors to vote for guilt in an otherwise close case.
Opening the door.
Oftentimes, a testifying witness unwittingly opens the door to evidence that the jury would not otherwise hear. Evidence of an accused's criminal history is generally inadmissible unless used to impeach the witness's credibility. Thus, such evidence would only be heard by the jury if the accused took the stand. Also, certain seemingly innocent statements can come out during testimony that can sink an otherwise winning case. The classic example is the DUI defendant who testifies and is asked why he did poorly on the roadside tests and responds, "I couldn't have even done them if I was sober."
Listen to your lawyer. The criminal defense lawyer is schooled in law and strategy and is in the best position to properly advise the client of the merits of testifying or remaining silent on a case by case basis. There are many considerations that the lawyer will discuss with the accused. While the ultimate decision rests with the accused, it is always good advice to listen to your lawyer and make an informed decision.