My take (and I don't have most of them, so it is not first hand experience).
Empower - good as talent and crest. Several heroes (PD, Athene) consider it top tier for them, others can benefit from having it as a crest
Sharpen - meh. Probably only great on HB as talent.
Feint - Sort of WG ish. Not sure combination works. WG is all about going all offensive, so it is not usually put on heroes that get attacked a lot, which is what dodge is for.
Brute Force - generally another WG clone, trading lower percentage for bonus crit damage. Good on heroes that have high rate of attack.
Iron Will - WG, except that you trade increase for duration of the fight for protection during start of it. Generally that means Mino, and probably not anyone else.
Regenerate - supposedly OP on PD. Also I heard of some builds where regenerate and LD crests make effective defensive/offensive combination.
Sacred Light - only slightly less protection than SS combined with good HP boost make this great tank talent and makes SS obsolete. Except that SL is now in $100 pack and SS is in $10 pack.
Unholly Pact - all offense, and probably WG clone of choice for anyone who wants WG. In particular, great on offensive Dove build, since she doesn't suffer from 20% damage taken.
Now, onto questions.
~What affect (if any) have these new talent had on the older ones?
Primarily several WG replacements as talents. And if your hero was always unhappy that WG talent and WG crest don't stack, now he can double down.
~Which will be the most popular?
SL is a clear winner, IMO.
~Which are least desirable?
Not sure... probably Sharpen.
~Where would you apply them & how? (Hero/Crest/Artifact/Pet/Traits)
Well, everything is a combination, and replaceable elements like crests, artifacts and pets are dependent on non-replaceable one and on modes. E.g, Lava vs GW vs AD would require different builds or at least different crests
A little too extensive a subject to cover, especially w/o personal experience.
~What are the disadvantage of using any of the above crests.
Not sure why crests; rest of topic is more focused on talents. But, IW disadvantage is obvious for long lasting heroes. UP is risky on low HP heroes; while they get increased offense, taking increased damage can kill hero fast, and dead hero doesn't have much offence.
Anyone who feels like I am completely wrong, or has actual experience, feel free to contradict me. A lot of what I wrote is theory or second hand info.