Who is Obligated?

The very first Mishna in Chagigah discusses those who are obligated to make the thrice-yearly pilgrimage to the Beis Hamikdash. The Mishna clearly states that the rule is that everyone is obligated except those specifically listed. Tosfos comments that the rule is that one cannot follow every rule. Tosfos notes that the Gemara (4a) lists a few types of professionals (e.g., a tanner) that are exempt from aliyah l’regel. Yet, those exemptions are not listed in the Mishna. Hence, Tosfos surmises that the rule is that every rule has unlisted exemptions – even this one.
One possible requirement to be obligated in aliyah l’regel that is not listed in the Mishna is that one must own land in Eretz Yisrael. To be sure, this requirement is the subject of much debate. (See Pesachim 8b, and the Meiri and Tzlach there.) However, one cannot prove either side of the debate from our Mishna, because Tosfos says that there are unlisted exemptions. Perhaps, one who does not own land in Eretz Yisrael is, in fact, exempt, yet the Mishna chose not to list it, just as it didn’t list a tanner as being exempt.