LOTS OF SPOILERS TO COME - YOU HAVE BEEN WARNED.
There is still lots of humour in it - some great Thor moments (putting him with GOTG crew was inspired), some great Bruce Banner moments - but intense scenes as follows:
One significant and one main character (real fan favourite) die quite brutally very early on in the film - in the first five minutes or so. The scene is a dark post-battle one and they are killed pretty mercilessly.
Thanos sacrifices his daughter to gain one of the infinity stones in quite a protracted, moving scene: you see her death.
We see Star Lord’s grief and rage when he finds out about her death.
There is a long battle scene with some quite intense fighting and ugly monsters (Lord of the Rings style battle in scale and action).
Towards the end, a number of key characters die, including a very moving scene between Vision and Scarlet Witch, and one between Tony Stark and Peter Parker.
There’s quite an unsettling flashback where you see the moment Thanos meets Gamora when she’s a little girl, and her whole village is killed while he distracts her.
So that gives you an idea of what to expect. It certainly is darker and crueller than the earlier films, especially in contrast to GOTG and Thor: Ragnarok, and the ending leaves you with a sense of - what?! Obviously, because part 2 is still to come, everything is left very much up in the air and inconclusive, with what seems like most of the various crews wiped out.