1) "And in this mountain"--likely a reference to Mount Zion. The prophecy likely finds its eschatological fulfillment with respect to figurative Zion (Revelation 14:1-3).
2) "a feast of fat things" for all the peoples. John N. Oswalt explains:
"a feast of fatness. To a people who did not have to worry about cholesterol, the fat portions of the meat were the best (Ps. 36:9 [Eng. 8]; 63:6 [Eng. 5]). Thus it is not surprising that these were the portions of the sacrifices reserved for God (Lev. 3:3; 4:8, 9). But here God is giving the rich food to his people, as the host (Ps. 24:6).²⁸ This is always the principle of sacrifice. God asks that we give to him in order that he may give to us. (Cf. the thank offering and the feasting associated with it, Lev. 7:11–18)."
See Oswalt, The Book of Isaiah, Chapters 1-39.
3) "a feast of wine on the lees . . . of wines on the lees well refined"
Oswalt offers these remarks:
"a feast of lees refers to wine which has been allowed to strengthen by leaving the dregs in the wine after the fermentation process. The wine, strained before drinking, was clear and strong, i.e., good wine. šemārîm, 'lees,' was probably used here also because of its assonance with šemānîm, 'fatness.'"
LXX: καὶ ποιήσει κύριος σαβαωθ πᾶσι τοῖς ἔθνεσιν ἐπὶ τὸ ὄρος τοῦτο πίονται εὐφροσύνην πίονται οἶνον χρίσονται μύρον
Brenton: "And the Lord of hosts shall make [a feast] for all the nations: on this mount they shall drink gladness, they shall drink wine:"
Wildberger has a long note on some textual issues and the literary form of Isaiah 25:6-8. See Hans Wildberger, Isaiah 13-27, Trans. Thomas H. Trapp (Minneapolis: Fortress Press, 1997), pages 523-531.
To summarize Wildberger's comments, he views Isaiah 25:6-8 as a coronation meal in which YHWH is host and offerer of divine grace. It is significant that Jehovah is called YHWH of hosts: the expression is used only two places in Isaiah 24-27. The prophet likewise distinguishes this meal by saying it is for the peoples, that is, the nations (non-Israelites). So God is extending his graciousness to all peoples.
Wildberger claims that Zion is "the focal point of Yahweh's rule, just as it is clear that the מלכות ׳הוה (kingdom of Yahweh) is not a transcendent type of rule."
He possibly means that the kingdom of YHWH is not heavenly, but he explicitly states that the "people of God" are not necessarily barred from this feast, by whom he probably means Israel. Although, see 1 Peter 2:9-10.
The commentator, Wildberger, concludes this section of his commentary about Isaiah 25:6-8 by contending:
"When סמנים (rich foods) is used (this plural form occurs only here), one ought not think it refers to meat, but rather to foods that have been prepared using a lot of oil. For an extraordinary festival, one also needs to have
an excellent wine, in this case one that has been prepared carefully, with special effort to strain it meticulously so as to make it clear, without sediment. What is described is the opposite of what was presumably the popular notion
in Israel, that Yahweh would prepare for the peoples a drinking feast at which he would 'make them drunk, until they become merry and then sleep a perpetual sleep and never wake' (Jer. 51:39; cf. Jer. 25:15ff. and Ps. 75:9)."
He also encourages readers to view this ancient text through Israelites eyes of antiquity.
Alec Motyer makes an interesting observation as well:
"The rich food and finest of wines contrast with the bread and water of 21:14. Aged wine can mean the sediment that forms in the process of fermentation (Zeph. 1:12), but here it means the wine itself, purified and matured by being allowed to stand. Finest of wines is (lit.) ‘lees thoroughly filtered’; best of meats (lit.) ‘rich food, filled with marrow’, a picture of nourishment."
See Genesis 4:4; Exodus 29:2; 7, 21, 23, 40; Nehemiah 8:10; Isaiah 28:1, 4.
Keil-Delitzsch prove to be worth consulting here. At least part of what they write appears to be on the mark:
Shemârim mezukkâkim are wines which have been left to stand upon their lees after the first fermentation is over, which have thus thoroughly fermented, and have been kept a long time (from shâmar, to keep, spec. to allow to ferment), and which are then filtered before drinking (Gr. οἶνος σακκίας, i.e., διΰλισμένος or διηθικὸς, from διηθεῖν, percolare), hence wine both strong and clear. Memuchâyı̄m might mean emedullatae ("with the marrow taken out;" compare, perhaps, Proverbs 31:3), but this could only apply to the bones, not to the fat meat itself; the meaning is therefore "mixed with marrow," made marrowy, medullosae. The thing symbolized in this way is the full enjoyment of blessedness in the perfected kingdom of God.