Here are a few trends I have noticed in 5ed that set this version apart from previous editions. These are related to the rules moreso than how we currently play the game but it all ties together.
* The Ubiquitous 3++ Invulnerable Save
This is a really big one. Space Marine armies now have access to stormshields that confer a 3++ save versus both shooting & close combat attacks. Space Wolves & Blood Angels now also have access to this wargear. The combination of the stormshield with a deathstar unit (see below) is extremely powerful and has completely changed the game and how we play. I have had my share of games where my opponent made an inordinate amount of 3++ saves; one good example was in a team game one of our opponents made over 20 3++ saves with his Wolfguard Battle Leader over the course of several rounds of close combat & destroyed an entire unit of Blood Crushers. If your opponent is rolling very hot you are going to be in for some long drawn out combats while they soak up all your damage. I'm not against the 3++ save but in terms of game effect I think it tends to be a bit too good at times. There is a geometric leap when you go from a 4++ to a 3++, just like 4+ versus 3+ but even better since it's always there.
* Deathstar Units
A deathstar unit is one that can basically take on an entire army and win. I think it started with Nob bikers, people then saw the potential and figured out how to apply it to other armies. Sure these units cost a lot of points but they are well worth it in terms of effectiveness alone. Most deathstar units consist of fully complex units with multiple wound models. They are fantastic in close combat and due to their nature they can absorb an ungodly amount of damage and keep pounding you hard. A deathstar unit does not have to be built around a fully complex unit, for example:
Captain Lysander + Vulkan + 5x assault terminator (thunderhammers & stormshields) + Landraider Crusader
Lysander & Vulkan can soak wounds from shooting, saving the terminators from potential unsaved wounds. Vulkan master crafts all of the thunderhammers plus Lysander has 4W, is an Eternal Warrior & has a S10 thunderhammer that inflicts +1 damage on the armor penetration table. You will have to score many wounds to get through all the stormshields & they are pounding away on you with those master crafted thunderhammers. I've watched this unit tear through entire armies.
I play another deathstar unit in my Khorne daemon army - the Skulltaker on a juggernaught, Herald on a juggernaught plus a large squad of complex juggernaughts. I phased out a Necron army with this deathstar with one charge. This is one of the best deathstars in the game (my opinion). I have seen another player who added two more Heralds also mounted on juggernaughts as well. This is one of the slower deathstars but since you can deepstrike off of an icon that's not such a big issue.
* Feel No Pain Across the Army
Blood Angels can do it now. So can Tyranids & you are in for some serious problems when a horde army has an additonal save that is better than their armor save... It's really crazy! The developers are handing this one out like candy too.
* Fully Complex Multi Wound Units
This has already been partially covered above in the entry for deathstars. Again you have a unit that can soak up vasts amounts of punishment. As already stated when included as a feature of a deathstar unit you have one really nasty unit!
So SW have access to Thunderwolves, fully complex with stormshields & joined to a multi wound character that is just vicious in close combat. Obviously Thunderwolves are extremely popular and thank goodness they don't have access to Feel No Pain.
If you are a competive gamer you must be able to effectively handle units such as deathstars. I think for the most part deathstars came about in 5ed. These features are extremely challenging. Basically the only effective way to deal with them is to inflict a high number wounds which is often not an easy thing to do. I am currently very much back into Blood Angels again and while they don't really have access to deathstars. (besides the Sanguinor & Mephiston) my basic counter to deathstars is to bring highly mobile combined units that have a very high offensive output. Hit the deathstars with so many attacks they are bound to fail some saves. If you do it right you can kill the bulk of a deathstar before it can counter attack.
Lastly I feel I must touch on IG mechanized armies. This is an army that is currently dominating the game. Most of us have heard fearsome tales about the Leaf Blower IG list. It's kind of cool but then again I know it's only a matter of time until some dimension in the game is modified & then IG Mexh will no longer continue to dominate. Everything changes if you wait long enough.