Regarding Milad, Shaykh Yasir Qadhi had an excellent response to those that celebrate it:
The most common source and evidence that is used by the people of innovation is 'So-and-so said.' As we pointed out in class, the opinion of an 'alim or mujtahid, however great, does not take the weight of an evidence in and of itself.
Subhan Allah, the scholars of the other three mad-habs disagreed with Imam Malik when he took the practice of the people of Madinah a hundred years after the Prophet salla Allahu alayhi wa salam as an evidence - how then can we take the opinion of a scholar centuries after him as an evidence?! And the schools of Islamic Law do not consider the statement of a Companion to be a source of Law - how about a mere scholar?
The fact that major scholars differed about an issue does not mean that we cannot criticize it. If we were to start listing the opinions of the scholars throughout the centuries that went against the clear ahadeeth, the list would indeed be a long one. The scholars insha Allah have their excuses (as I'm sure Sh. Yaser Birjas has discussed in his TCE class in great detail), but when the truth becomes clear to us, let us beware of blindly following a human being in opposition to the Quran and Sunnah.
Also, with regards to bid'ahs that lead to shirk (such as the mawlid; in fact it is correct to say that most mawlids around the world have elements of shirk in them - remember the lines from the Burdah of al-Busayri?) we have to be even more strict.
One last point. Realisitically, the types of mawlids that Ibn Hajr, an-Nawawi and scholars of their calibre sanctioned are rare to find. Read what they said: Ibn Hajr mentions praying and reading Quran; Nawawi mentions giving alms and enjoining the good, etc - where is this from the dancing and innovated group dhikrs (not to mention acts of shirk) that occurs in most mawlids around the world?
To conclude, the mawlid as it is practiced is a clear bid'ah, and a blatant stepping stone to shirk (and, in many cases, manifestations of shirk).
When you understand the meaning and psychology of 'ibaadah (love, hope and fear), you will find that practicioners of the mawlid demonstrate all of these emotions far more perfectly during these innovated acts than they do in their salat and du'aa to Allah, but unfortunately during their ceremonies these emotions are not being directed to Allah. And this one issue, in and of itself, is enough of an indication of the dangers of the mawlid.
The Prophet salla Allahy alayhi wa sallam was sent as a mercy to mankind, so that mankind worship Allah, and not so that he become the object of man's devotion besides Allah. True love towards him is shown by following his sunnah to the best of one's abilities, and not by disobeying him and inventing acts of worship that he himself or his noble Companions did not sanction.
We ask Allah because of our love for His beloved Prophet and our defence of his pure Sunnah that He bless us with the Prophet's shafa'3ah on the Day of Judgment... Ameen!