Oh, I figured you wanted the general outline since you said "changes and releases", but ok. I have too much spare time today anyways
Skitarii and Cult Mech are both solid armies, Skitarii in Flesh Tearers (
BA) drop pods can be a really brutal alpha strike with their canticles boosting their
BS. There is also a formation where you take Skitarii, Cult. Mech and a Knight, and get free upgrades and no Gets Hot! results. It placed second at the
BAO this year, if I recall right.
The new Space Marine and Dark Angels formations are really powerful, with units getting free transports! With ObSec! This naturally leads to some very powerful
MSU builds, and a lot of armies will struggle to grab objectives against such an army.
Daemonkin is a fun, fluffy and semi-viable addition to Chaos forces. Being able to join
CSM characters to daemon units (no Daemonic Instability!) is really valuable. Just throwing
msu units at your opponent and getting Blood Tithe points, which can then be used to summon new units or boosting your army, is both fun and thematically fitting to Khorne, as well as a pretty viable strategy against armies that can't handle the pressure. You also rack up blood tithe points fast against
MSU builds (like overmentioned space marine formations).
Necrons are very, very durable and thus very strong. Wraith-spam, lychguard deathstars, silver tide, av13 spam and even flayed ones are pretty solid builds.
Eldar are currently the top dogs in most people's eyes (together with
SM and I suppose
DA), with D-weapons all over the place. Eldar Jetbikes are also ObSec and now every model in a unit can take a scatter laser. So they have a lot of punch, insane mobility, decent durability (3+ and jink) and obsec, and cost less than Possessed Chaos Marines..yeah. And Wraithknights are gargantuans with D-weapon access.
Harlequins are cool and can have some neat builds, but I feel like they were swept away by the Craftworld Eldar (which are so much stronger) and you don't see them often.
Imperial Knights got some new options.
My personal favorite release has been Renegades & Heretics from ForgeWorld: like
IG, just better at doing horde+artillery.