I was a Warhammer/40K player for close to 20 years now (not anymore but that's another story) so i'll give it a shot.
To put it bluntly.. the movie sucked.. hard. When i watch a TV show or a movie (usually at my computer) and start looking at other websites without pausing what i was watching it usually means i lost interest and this happend quite frequently during Ultramarines.
Technically it was mediocre to say the least. The animations were stiff, the general vibe was a bit soulless and it just didn'r click.
The story was boring as hell.. rookies need to investigate a mystery while led by 2 veterans who spout off the usual one liners to show they have experienced more.
For most of the movie they are just wandering through the haze and nothing happens.. it got boring real fast.
Then we have their enemies.. Chaos Marines. To put it into perspective.. Chaos Marines are the evil equivalent of Space Marines who broke their oath to the Empire 10.000 years ago which resulted in a huge, galaxywide civil war which could only by stopped by huge sacrifices and losses.
So they should be the equals of Space Marines at least.. equipment wise, skills and tactics yet here they attack mindlessly towards the Ultras getting shot down because they rush at them over open ground
Perfect example of the Stormtrooper trope and it had me very disappointed.
Then the storyline with the possession of the Captain.. been there, done that a thousand times over.. boring.
That movie was awaited by the community with great anticipation.. up until then we only had small minimovies out of games and the occasional pre-rendered cut scene from these so a full feature movie was something very exciting.
Sad to say they dropped the ball on that one and only due to story.. i get it that they probably didn't have the funds of a major hollywood studio but there's little excuse to have such an uninspired and boring movie.
The 40K universe has such a rich background and it was nowhere to be seen.. there's so much cool in it that you could have drawn up one heck of a story but we got something that seemed to come from a beginner. Wasted opportunity that movie.