I think it's fair to say that my interest in Warhammer 40k 8th edition peaked early and dropped off a cliff after playing a few games. I don't think I really gave it a fair whack, something I think I've at least started to do better with with 9th edition (and Middle Earth Strategy Battle Game).
I have played exactly one game of 9th (and MESBG) this year and both were brilliant. As is my want I've spent more time analysing why I think that than actually playing the games.
1) Terrain: playing on a table dense with fully painted terrain is so much better than not. MESBG seems to require lots of terrain and 9th's changes to line of sight make terrain matter more (at least to me). Having them look good at the same time makes games cinematic.
2) Fully painted armies: much like terrain it just makes the whole thing pop and avoids it feeling half-assed. I can't see myself ever using an unpainted army ever again.
3) Actually read and internalise the rules: in 8th I just skimmed the rules, made lists with Battlescribe, and relied too much on the groups book nerd. Become the book nerd. Watch battle reports and try to work out what people were trying to achieve. Pay attention ya big idiot.
4) Play the mission: most of my games of 8th really were just elimination games where we blasted each other off the board. Actually playing with objectives/missions adds a layer to the game that I honestly never really paid attention to, even as far back as playing LotR as a kid.
5) Imperial Guard: finding an army who's play style matches yours is key. I started a Cadian army over the pando but last weekend was their first outing. Turns out tanks are brilliant fun and the Order mechanic is really fluffy and powerful.
I'm looking forward to playing more Warhammer in the new year (hopefully). I'm aiming to bring the Cadian force up to 2000pts.