I didn't see any relationship development with Garrus. In ME2, he's suddenly Shepards best friend, no matter how you treated him. "There's no Shepard without Vakarian" is one of the worst lines in the series for me.
I also disagree that Liara has a massive character shift without an arc. ME1 Liara was a smart, but inexperienced, a young asari who had spent her life mostly out of touch with the world because she was too busy with her studies. Enter Shepard, the experienced, battle-hardened warrior, who first rescues her and then turns out to have a connection to the Protheans, who Liara has always been fascinated by. It makes perfect sense that she would fall for Shepard, male or female.
Then he/she goes and dies on her. For a sensitive and vulnerable person like Liara, her first love dying a violent death practically right before her eyes would be incredibly difficult to cope with. So what does she do? She finds a new obsession with the Shadow Broker (obsessive behavior being a typical trait of hers, see her previous obsession with the Protheans) and completely buries herself in it to drown out the pain. And between devoting her life to hunting down the Shadow Broker and having lost the only person she's ever loved the way she did, of course she becomes somewhat jaded and cynical.
The big change is between the first and second game. In the first, the relationship is definitely mentor/student. it's why I love the paragon option on the Dr. Saleon mission. In the second, they are more friendly, though I don't get the "best friend" vibe until the third game. I really like that Garrus talks with Shepard about the missions. It makes Shepard feel more like a person.
For Liara, the problem isn't that she is cynical; it's that she is a violent rage machine. The connection to Shepard for this shift isn't well explained. That's part why the LotSB DLC was so good. That change in Liara needed to be explored.
Add to this the whole "Tessia is lost, OMG! It was the most important planet in the Universe, now I want to kill myself" while your Shepard could (and for many of us would) feel nothing special about it.
Forced emotions is a bad thing, probably worse than forced lost battles (a la Kay Leng).
Well, if I recall, Shepard always feels bad about Thessia whether or not you care.
As to the second part, that's the problem with the intro. "Oh, some kid died.You should feel bad"
Not to all characters, no, but to some of them. And Liara indeed got the most attention - special DLC dedicated to her alone! Hence, more hate than for others.
Not to mention, different people hate different things. Traynor irritates me more then Allers or Liara, yet, she is generally loved character.
I think that DLC was needed to explain the character shift. It does fit best if Liara was the LI from the first game.
Liara is one of the fan favorites as well, both according to Bioware's own metrics (she was the most used squadmate) and the largest fan survey. The difference between Liara and Garrus and Tali is that she's a more polarizing character than the other two. Fans tend to either love or hate her, and those that don't have any strong opinions on her one way or the other appear to be a minority.
To some extent that's also true of Tali, but not to the same extent as Liara. Garrus is a bit odd in that it seems no one ever hates the character for getting a lot of love from the writers, whereas both Tali (ME2) and Liara (ME3) got complaints about the amount of content or dialogue they received.
One other thing I noticed...
Both Liara and Tali were criticized for getting the canon friend treatment, whereas Wrex, Garrus, and Grunt are not. It seems fans object to the player not being given a choice on whether or not their character is close to a squadmate only if the squadmate in question is female.
Oh yeah? I hadn't seen that and was going off what I'd read from people's personal opinions. As for why Liara and Tali get it more, I'm guessing there are two reasons.
1) Shipping. Fans of one hate the other. Particularly with (I assume) most players being male and male Shepard being played 4:1 (last I knew) over Femshep.
2) They are similar in character in the first game. Both are young, naive, non-military, and impressed by Shepard.
I hear many complaints about the "forced friendship" between Shepard and Garrus. Vilio01 doesn't like it. As for Wrex, that develops over the first game if you want it to. Wrex can also die. After that, he's not in the squad. His attitude toward you will actually be based on what Shepard does. Grunt can stay in his tank.