I'm not the original poster, but for me it is:
Technical: The game is not attractive visually. It isn't locked at 60 FPS (on the Xbone) and it tears like crazy. I could forgive Respawn a little if the framerate was locked and there was no tearing, but as it standings it is an ugly game with performance problems. Compared to anything on the PS4, this game is visually laughable. Hell, even compared to Garden Warfare this game is fugly.
Time to death: Pilots simply die way too fast in this (and it is why I despise CoD). The speed of the movement is great, but being killed almost instantly and not being able to really react to anything is boring. Both defensively, but also offensively. I don't feel accomplished when I shoot at a player for half a second and they die without even being able to tell they were under attack.
It also prevents things like getting stomped on by a Titan from feeling impactful. If someone dies instantly from anything, there is vary little variation in how powerful stuff feels. Getting nuked by an exploding Titan feels the same as being shot three times by a Smart Pistol, and happens in about the same amount of time.
Leveling: I hate leveling in shooters and not having access to weapons and abilities. It just feels like a grind to get to the weapon I want, especially in a game like this where you're not unlocking new variants on a type but the entire category of weapon because there is only one of each type.
The only thing I really enjoy about this is the pilot movement and Titan combat, but mostly because Titan engagements actually feel meaningful because they don't instantly respawn and take time to kill. If this game had Titan only CTF and Hard Point modes on maps designed for 12 Titans, I'd love this game.
Time to death:
The only online FPS games I have played are: BF3, BF4, Hawken and now Titanfall. Hawken is somewhat different so instant death cannot occur. From my experience you die pretty fast in Battlefield if someone has a modicum of aiming ability (perhaps I am just rubbish).
In Titanfall there have been many times when I was being shot but have used my running abilities to evade death. It is rare if someone can aim with any level of ability but you can get out of a near-death situation by using parkour (the stim ability is very handy).
If you get shot in the back then you would have died in any game as you were oblivious to what was going on. Front on shooting can be very frustrating as I have often shot someone first (in my interpretation of the gaming world) and yet on killcam it suggested otherwise. I guess that any FPS game would result in the same outcome, though, and that is death. I can always tell players who are playing the game to its strengths e.g. moving a lot and using the environment to scale and run through as they are more difficult to shoot due to being a constant moving target. Some players just stand still and on the floor which makes them easy pickings.
You say about players not knowing they have been under attack and die without being able to respond. To me this leans towards situational awareness. Of course you cannot see all that is around you all of the time. But of all the abilities employed in playing an online FPS game I feel that knowing what is around you at nearly all the time is the second most important; aiming is probably the most important!
I could say that being shot by any form of weapon in any form of online FPS is essentially the same. I have been pistol shot, rocket launched, grenaded, stabbed, machine gunned and they have all ended in death in the same way.
Leveling:
I am not a massive fan of leveling but most guns open up fast and you unlock the extras to that gun if you stick with it. I never felt I had to grind to get unlocks as it all occurred very naturally just by playing not that many matches. I am not 'regening' (thus not losing my guns) as I don't want to get involved in the do-this-100-times meta game of the challenges.
Anyway, good insight into why the game hasn't clicked for you. Too many people just say they do not like a game without going into detail.
