The main differences from mid 2000's to the current game, besides the Flyers, Giant robots and Gargantuan MC is the game is more deadly in all phases (except moving >> and that has more movement lol).
Psykic: Mainly buffs to make units much more powerful/tougher but there are some damaging powers. Or conjurations to get more units.
Shooting : many newer weapons have been added often with multiple profiles for better shooting at different targets. AP3 and AP2 firepower is more common so marines and termies are not safe either. Multi shot str5-8 has also become more available so just putting mass wounds on and watching them fail saves is a valid tactic in lists as well.
Assault: is way more deadly. Everyone in 2" gets full attacks and there are pile in's at every init step so a lot of models get to attack. Characters being able to challenge can also crush out some of a basic units h2h power too. (eg Init5 IC killing out PF sgts is big now). Assaults are usually over in 1-4 phases now there are no massive scrums that go on for an entire game (6-12 phases...).
It is usually harder to get into assault now (with overwatch fire before the charge) but a well planned assault or a list designed around it can butcher entire flanks much quicker than it used to. Almost as fast as a BA rhino rush in 3rd ed...
Where I am Maelstrom objectives rarely get used. They are all just a bit too random.... On a scale of say 4-12 point is an average game, one player can be 6-0 up in turn 1 (litterally!!!!!! the worst game I have seen on random draw cards was a 9-1 by the end of turn 1) The game is effectively over. Not terribly fun for either player on just a random card draw
. Play out the next 1hr and 50 mins for dice rolling. (with a good mate I play alot of games with if games become very onesided we call them at the end of a turn and just reset the game).
A more limited version of Maelstrom (the ITC has a more balanced system) has been used occasionally and it is a nice balancer to std game conditions. (obj=3pts, and the 3x 1pt std objectives (kill warlord, get into enemy deployment, first blood).
Cheap tarpit units that are made fearless (eg 20-30 cultists with an apostle) are often the answer to many death stars and giant things.... they usu manoeuvre for 1-2 turns then assault. if your tarpit can block/assault them in turn3 and hold them up for 2-3 turns(3-5 phases) you will have basically taken them out of the game. They will get out and maybe do 1 thing after that.
40K has not really changed that much. The more deadly game takes a little getting used to but it still requires the same strategic and tactical game as before. I find I can still put myself at and advantage of alot of opponents in deployment (still) and they are then still playing catch up (tactically). As long as you trade damage/units well you still end up ahead and hopefully on or near objectives. Just bear in mind movement of most things is alot faster (eg eldar jetbikes can move48" in 1 turn hell even a humble rhino can move 18" if it doesn't shoot to contest objectives) So positional play is key.... know where every unit can be by turn5.... don't wait around and then find yourself 2 movement phase away from an objective when you need to do it in 1...
So many players in the newer younger generation are still damaged focussed. It is always "look how much damage this unit can do....!!!" They throw away a 300pt unit to do its damage. If you can make the unit it destroys only a 150 pt unit (before you pulverise it) you are ahead...
and yes CSM are definitely not near the top feel free to take whatever.