I find some four characteristic features of an Ecumenical Synod here and there referred to by many authors….
1. The chief distinguishing feature of all Ecumenical Synods is the fact that they are convoked at the behest, not of the Pope or of such and such a patriarch, but by imperial orders, i.e., at the behest of emperors or kings….
2. [They are] for the purpose of discussing matters of faith, and consequently to render a decision, and give it dogmatic definition….
3. All dogmas laid down by them and their canons [are] to be orthodox, pious, and in agreement with the divine Scriptures or previous Ecumenical Synods….
4. All Orthodox patriarchs and prelates of the catholic Church [are] to agree and to accept everything that has been decreed and ordained by the Ecumenical Synods, either by their personal presence or by their own legate, or deputy, or, in the absence of such a representative, by means of a letter of their own.
– St. Nikodemos the Hagiorite in The Rudder (Source)
This is very interesting in that the Protestant can in principle accept these criterias for an ecumenical council, but we merely treat criteria 3 as the “sudden death” or critical criteria. Criteria 3 is the only criteria of significance, failing that, the rest are simply rendered irrelevant.