Gemara

Rav Sheshes: One who disgraces the festivals is like one who clings to idolatry - right after "Elohei Masechah Lo Sa'aseh Lach" it says "Es Chag ha'Matzos Tishmor".

Avos 3:11 If one profanes Kodshim, or disgraces the festivals...even if he has Torah and good deeds he has no share in the world to come.

Rishonim

Rambam (Hilchos Yom Tov 6:16): Just like it is a Mitzvah to honor Shabbos and make it delightful, the same applies to Yom Tov...If one disgraces the festivals it is as if he clings to idolatry.

Source: The Yerushalmi (Pesachim 6:2) learns from the Mishnah (one who disgraces the festivals...) that it is a Mitzvah to eat nice food on Yom Tov (as opposed to Chol ha'Mo'ed).

Rashbam (DH veha'Mevazeh): He disgraces the festivals by doing Melachah on Chol ha'Mo'ed.

Source (Torah Temimah 34:20): Chagigah 18a expounds "Es Chag ha'Matzos Tishmor" to refer to Chol ha'Mo'ed.

Rebuttal (Tosfos Chagigah 18a DH Cholo): The Rashbam holds that Melachah on Chol ha'Mo'ed is forbidden mid'Oraisa. Really, it is mid'Rabanan. If it was mid'Oraisa, Chachamim could not permit things such as Devar ha'Eved.

Rashi (Avos 3:11 DH veha'Mevazeh): The disgrace is Melachah on Chol ha'Mo'ed, or eating and drinking on Chol ha'Mo'ed like on a weekday. Because Chol ha'Mo'ed is not as stringent as Yom Tov, he is not concerned for it. This is "Ki Devar Hash-m Bozah".

R. Yonah (Avos ibid.): It says 'disgraces' here (in Avos) and 'profanes' in Pesachim because here it discusses Chol ha'Mo'ed, and in Pesachim it discusses Yom Tov. It would not need to teach about Melachah on Yom Tov itself - that is like Chilul Shabbos!

Note: Our text is 'disgraces' in both places.

Me'iri (Avos ibid.): Most Mo'adim recall Hash-m's miracles. Shabbos is written amidst the Mo'adim to teach that one who disgraces the Mo'adim (denies Hash-m's miracles) will come to deny Divine creation. Kefirah is the only reason one would disgrace the festivals, for there is no Yetzer ha'Ra to do so.

Maharal (Chidushei Agados Makos 23a): "Mo'ed" connotes a meeting. They would lift the Paroches on Mo'adim to let Benei Yisrael see the Cheruvim embracing, to show Yisrael's dearness to Hash-m. Disgracing Mo'adim shows a preference to cling to something else other than Hash-m.

Maharsha (ibid. DH Kol ha'Mevazeh): The Mo'adim in their time testify to Yetzi'as Miztrayim. In order to change the Mo'adim, Yarav'am led Yisrael to serve idolatry.

Poskim

Rema (OC 530:1): Chachamim saw fit to permit certain Melachos on Chol ha'Mo'ed.

Magen Avraham (1): The Rema forbids Melachah on Chol ha'Mo'ed mid'Oraisa (otherwise, he should have said that Chachamim forbade certain Melachos), but the Torah authorized Chachamim to decide which are forbidden.

Kaf ha'Chayim (1): The Beis Yosef brings several Poskim who forbid Melachah mid'Oraisa. Poskim argue about the Beis Yosef's opinion.

Mishnah Berurah (2): Frivolity on Chol ha'Mo'ed is a worse Isur than Melachah! The purpose of the Mo'adim is so Yisrael should eat, drink and learn Torah vigorously!