Interesting OP, Kev.
Some thoughts...
I always view "self-indulgent" as playing not for what the song requires, but to feed an ego. It's a very, very fine line, and many bands cross it without even intending to. It is, as is much in life, a very subjective adjective. I mean, to play devil's advocate, who is the one who decides what the song requires? The writer, or the listener? Both?! So, generally speaking, in a vacuum, it's just that personal feeling when you think as the listener that something being done isn't meant to serve the song, but to serve the ego. And where that line is, and who decides, obviously, differs among us all.
Regarding "sounding dated," that one is a bit easier, at least for me. Most of us here are music snobs. And it took a while for us all to develop a sense of recognizing songwriting and performance, and separating that from the production value a song has based on when it was recorded. I mean, compared to some of the brilliant mixes today and the technology, Led Zeppelin songs sound like dogshit!
Fuck, so does early Rush!
But we know, as music nerds and snobs, after many years, that "dated" production sound is just the byproduct of the era in which something was recorded.
Of course, that's "dated" relative to the production, not necessarily to the style of song.
I mean, yeah, you hear a gang vocal with a poppy chorus in 4/4 time, with high vocals and a swaggy guitar solo with lyrics about tits and ass, you're probably going to guess the 1980s, and most of the time, you'd be right. I mean, but whether it is a negative adjective or not really depends.
Cool topic. Probably a lot of differing opinions.
I can tell you one thing, TAC is one dated fogey.