I love it because I invested so much of my time playing all the maps, finding all the choke points - building up madly powerful squads. I know I can do all of that in BF - but there is a different feel to TA. It isn't better than BF, just different.
BF is fast-paced scary xenos. TA is slow-paced scary xenos.
TA felt a lot more like survival by the skin of your teeth with you team of four. There are some levels in BF like that, but I definitely feel more like an elite strike team in BF where the xeno are reacting to me. TA was a nail-biter for whole levels at a time because I felt like I was consistently reacting to xenos.
Even in BF when something goes pear-shaped, my team has the capacity to handle it and adapt, bringing up reinforcement if necessary. Not always the case in TA.
The levels were great in TA - sometimes you felt confident you could split your team up to achieve different sub-tasks, whereas with other levels, you felt you had to have strong overlapping fire, and moved at a snail's pace because of it.
As much as heat is great in BF, I like the ammo counters and the melee weapon balance. You had to watch that ammo counter as it counted down, and had to ensure your plan-B (melee) was sufficient to get you to the end of your objective.
I'm with slayernz, the ammo game was always fun in TA. Couple that with the much more scary hits that the xenos can land on you (it's not uncommon for you to lose a Templar to a single hit early in the game), and TA had a huge amount of tension.
That's not to say that BF isn't tense, it is, but that TA had a different kind of tension. @rearick said it best, I always feel as if I have some possibility of overcoming a situation in BF and that is most definitely not the case in TA. In TA, sometimes you just had to sacrifice a couple Templars to pull your captain out of the fire.
Also, just like slayernz, I spent way too much time building up my squads and had some truly heroic squads near the end. It's one of those games that always has a place of honor on my phone.
The only thing I didn't like about the movement was that you couldn't do a sideways shuffle (you had to use one movement point to turn to face your new direction. Other than that, it was all very strategic planning type planning