Is there any issue with a man having long hair if he isn't a nazir, or its necessitating him to become one? Long hair is here defined as shoulder-length or longer.
When I was learning in Israel, someone posted an article written by R' Aharon Lichtenstein about having long hair. If I recall correctly, there were 3 issues he raised and evaluated:
1) Interruption for tefillin: There is a dispute between the Machatzis Hashekel and the Pri Megadim as to whether hair is an interruption on the spot it is grown, or only if it is spread onto that part of the head from roots somewhere else on the head. R' Lichtenstein concluded that the opinion of the Pri Megadim is reliable, although he pointed out that there is possibly the same issue of an interruption by the knot at the back, and it is very difficult for someone with long hair to not have it hanging down over the spot where the knot in the back will be.
2) Imitating the ways of non-Jews The verse in Vayikra 18:3 says not to imitate the ways of the Egyptians or the Canaanites. The Rambam in Hilchos Avodas Kochavim 11:1 writes that one should not resemble idolaters, and one example he gives is in their hair-styles. The Maharik writes that the prohibition applies to anything the non-Jews do for a non-utilitarian purpose. The Vilna Gaon writes that this prohibition applies to anything that a Jew does as an act of following non-Jewish culture, regardless of why the non-Jews do so. Accordingly, R' Lichtenstein entertained that this would constitute a Biblical violation according to both the Maharik and the Vilna Gaon. However, this would depend on assessing whether this is done commonly enough within Jewish society to consider it a purely non-Jewish mode of dress. But it is at least something to be concerned about.
3) Man wearing a woman's clothing: Devarim 25:5 states that a man shall not wear a woman's clothing. The Netziv, based off of the Sifri, says that there are two aspects of the prohibition, one of which is doing an act which is specific to the opposite gender. Rabbi Lichtenstein was uncertain if this was definitively something gender-specific to women, but concluded that it was at least possibly included, and therefore, again, a man growing long hair should be avoided to avoid a possible Biblical prohibition.
He concluded with a "spirit of the Law" type of discussion about the immoral roots the culture that spawned this hair-style (think Woodstock) and its association therewith.
Therefore, it's not such a good thing.
(If I find the article I will edit in more specific sources. Anyone else is welcome to do so also.)
See Rabbi Neustadt’s book which quotes the Mishnah Berurah 27: 15 to say that long hair is not a natural outgrowth of the body and therefore constitutes a chatzitzah between the head and the head-tefila. There are however lenient opinions.
There is a long article by Rabbi Aryeh Lebowitz - this is his conclusion:
While one who grows long hair cannot be said to have definitively violated any given prohibition, whether biblical or rabbinic, the overwhelming consensus amongst the leading poskim is that growing long hair is a practice that should be avoided both for halachic and meta-halachic reasons. Rabbi Ovadia Yosef concludes that it is worthwhile to give musar about this and remind people how a ben torah should present himself. Rabbi Shternbuch reports that when he visited Rav Ahron Rokeach (the Belzer Rebbe), the Rebbe asked him to tell the yeshiva boys that his father thought growing long hair was an issur deoraiso, and he obviously thought it was something worth correcting. Rabbi Shternbuch does note that when a boy comes from a family where he is expected to keep up with certain styles, and a sudden change in outward appearance is likely to upset the sholom bayis and may discourage the parents from supporting the boy’s development as a ben torah , it is certainly ill advised for the boy to cut his hair. However, even in these circumstances the ben torah should realize that the ultimate goal should be to appear as a ben torah and avoid the various concerns outlined in this essay.
The Shulachan Aruch explains that on Rosh Hodesh we relive tefillin at the time of Musaf, which is the Tefila of Keter, we say keter Yintu Lecha. The tefilin are also Keter, so we have to remove because we are eleveated to level of Keter.
Since the Torah gives a Nazir a level of Keter (R Moshe Koredeverk, RaMbaN, R Bechaye) “Crown of God is on his head” refers to his hair. Therefore the Nazir is prohibited to wear material Tefilin.