需要注意的是【xūyào zhùyì de shì】

需要注意的是 flags a caveat: a limit, an exception, a thing that could go wrong. It opens the sentence, followed by a comma and a full clause, and turns up in instructions, notices, manuals and teacher's explanations.

特别 strengthens it: 需要特别注意的是 "it is especially important to note that". 要注意的是 is a lighter spoken form.

需要注意的是 or 值得注意的是

需要注意的是 tells the reader to take care. 值得注意的是 "notably" points out an observation – a trend, a surprising figure – without telling anyone to act.

值得一提的是 is for a positive or interesting extra, not a warning.

Mistakes

The 的是 is part of the frame. Without it the phrase can't introduce a clause.

• × 需要注意，考试不能用手机 → 需要注意的是，考试不能用手机
• × 需要注意是，考试不能用手机 → 需要注意的是，考试不能用手机