So what are some examples of otherwise good songs with poor or overwrought solos? Within the canon of the best of metal I mean. I struggle to think of examples.
It can be hard to find fault in any aspect of top-tier material for the very fact that it is top-tier and that with whatever faults exist, it still succeeds overwhelmingly.
So I guess I'm only aiming for a general observation from a primarily compositional standpoint. My main objection would be to the verse-chorus song structure which typically employs soloing to tie off loose ends before repeating the opening riffs to close a piece. It's not necessarily bad just outmoded in its stuplicity.
The other appoach is to put some thought into the key melodic content while improvising over it accordingly. The example of Varathron is well apt and I would add to it Euronymous' classic Freezing Moon solo. Unfortunately it still kind of fits into the neat little 'time for a solo' mentality.
The example left to us on Hvis lyset tar oss
sees leads/solos thoroughly augment every element of riffing in song construction. Needless to say that album represents the farthest evolutionary point of the genre in almost any other element one could think of. Darkthrone's A Blaze ...
era had similar potential.