1. Rukia is so much older than Ichigo, and the problem between their relationship turning into romantic one is a question of maturity. Even 20 year old has problems with someone 16 year old, so over 100 years old would surely feel like 16 year old is too much of a boy to her. It's hard to get attracted on to someone who's practically a child compared to you. And after all, Rukia does time to time act condescendingly on Ichigo and it's commonly accepted thing in fandom that she's more mature. She's more likely to get interested in Renji, who is supposedly same age as her, or on Kaien, who was older than her.
Whoa, what an argument- it has more holes than Swiss cheese.
First of all, Kaien is dead. Second of all, even if he was alive, he was happily married to a woman he adored so much he died for her. And even if he lived after his wife's death, nothing would have happened. Rukia looked up to Kaien, and who wouldn't? He also treated her like a much loved disciple, and it was beautiful relationship they shared. Putting a romance tag on it takes away (according to me) most of its meaning to Rukia. She admired him deeply, and also his wife- it was the sort of reverential adoration a little girl shows her older brother/mentor/teacher. I don't think having a romantic connection with him ever crossed her mind.
Renji.. is so difficult. I think they could have worked at some point. That is, before he fake!rejoices for her getting adopted. You can see the desperation in her eyes, the silent pleading for him at least to protest.. but he didn't. I think her heart did break in a big way then. And now, I think she's understood why he behaved like that, and their relationship has settled into a warm friendship. I honestly think that is all it will ever be. According to me, even Renji seems to expect nothing more.
Coming to Ichigo, Rukia has never treated him like a child. Ever. I challenge detractors to name an instance, and am sure they can come up with nothing. She has always treated him like an equal, and she should too. Who repaired her relationship with Renji in the first place? Ichigo did. Who helped her make her peace with Kaien's death? Ichigo did. Before Ichigo, she was this self-blaming, lost person who was so broken inside. He healed her. I know some people like to claim that she's just his mentor and all that.. but he's been hers too. In ways that are subtler, but he has. He changed her life as much as she changed his, and that's why I think their bond is special. It will flourish.
2. While Ichigo was shown thinking about Rukia a few times during the recent arc, Rukia didn't show any hints of having missed him or thought about him during the time-skip. In fact, she has become a lieutenant and all, so it looks like she did quite well without Ichigo in her life. She did show how she believed in Ichigo, but that was something she has done before and something that others, like Byakuya, also show towards him. This argument further supports argument 1.
And this argument is flawed from the start. Just because something can't be seen doesn't mean it doesn't exist. We went through the 17 months from Ichigo's point of view. We have no clue where Rukia was, or what she did. We have no proof that she has not missed him. And if people think that her becoming a lieutenant is a sign of not caring then they're off their heads. She might have missed him as much as he did, as deeply, but that didn't mean she stopped breathing and instead hung around like a lovelorn maiden drenching a hanky with tears. Neither did Ichigo. They both got on with life, maybe not with the same zest, but they did do something. Rukia did, she was incredibly concerned when Ichigo heard her through the badge and she came running to help him. Both Ichigo and Rukia are people of action, not pretty words. They show what they feel.