I think this scene is perfectly played. I feel that Ennis just didnt pay that much attention to his daughter, not thro lack of love but because he didnt know how to.
What this scene shows is a change in Ennis' character, it's almost the moral of the story.
Previously, Ennis' relationship with Jack, paralellled that of his relationship with his eldest daughter: Both Jack and Alma had requested Ennis to change his way of life for them, both had been turned down (trying to hide the hurt, but it's obvious to the viewer at least). When Alma Jnr asks her Dad to come to her wedding, he says he cant because of work but then he remembers that he refused Jack more time because of work once before, the price he paid for this was that he would never see Jack again. This makes him realise his priorities and that his job isnt as important as giving time to the people he loves. He makes the connection with his daughter that she's always yearned for, it's so touching.
I agree with daveya26, in certain aspects, his relationship with his eldest daughter paralleled that which he had with Jack. An important scene which illustrates this 'parallel' is the following:
Iin the battered pickup truck with Alma Jr., driving her home from the outing that they had had along with Cassie...Alma asks Ennis if she could move in with him. (She makes up a pretty lame story about mom Alma and Monroe being awful strict on her...but look closely at her eyes...they reveal to us that she is telling a 'white lie'...)...'that' is not the real reason....she suspects...she feels that something is wrong...she intuitively senses Ennis pain and inner turmoil...she knows...(maybe not 'everything'...but, at least 'something'...) And it is her own small way, (a way of yet still a child)... of offering her father Ennis some help...
She asks Ennis if she can move in with him. Ennis refuses...
Quickly...almost too quickly...(feeling embarassed?) she says: "That's all right... I understand..."
Jack also asked Ennis to live together. But Ennis, absolutely frightened to the core of Jack's proposal, refuses him too.
So...we see: in that last scene in his empty, lonely trailer, Ennis ends up with two pieces of garments belonging to two of the people he loves the most... Jacks shirt from the mountain...and Alma Jr.'s sweater which she forgets and leaves behind when she drives off...
From Alma Jr. and Jack...the two people on this earth whom he holds/held closest to his heart...
With the one, he realises that he has made a terrible, terrible, and ultimately fatal mistake. But, with the other (which signifies "Hope"): he finally chooses to change...turn a new leaf...make a conscious effort...however late it might be...to open up this tiny and very small closed world of his, and to accept the hand that is reaching out to him, by that other human being that loves him dearly...
"I reckon they can find themselves a new cowboy...My little girl...is gettin' married." he has just said... A real effort by Ennis to change. Remember how he always refused Jack in the past? "I'm sorry Jack...I got the girls this weekend..." ... "I'm sorry Jack, I can't get the time off..." ... "I'm sorry Jack...it ain't gonna be that way..." ... "I'm sorry Jack, I'm not movin' to Texas..." ...
Clearly, Jack refuses to make the same mistake with Alma Jr. that he made with Jack...
So, in a purposeful manner showing us his deliberation to 'move forward' and not to repeat past mistakes...he lovingly folds Alma's sweater and steadfastly puts it away in the closet... (She will be back...I will see her again...)
But...when the closet door opens...we sadly see: Jacks shirt reverently hanging there, on the inside-back of the door, in rememberance of love lost...and past mistakes made...
This time, with Ennis' own white shirt now on the 'outside', and Jacks blue denim one tucked away on the 'inside'...as if Ennis is now trying to wrap his arms around Jack...feel close to him...keep him warm...comfort him...
In exactly the same gentle and loving way that Jack had wrapped his arms around Ennis, so many a time in the past...
Jack had kept his shirt wrapped around Ennis' for twenty odd years...secretly tucked away in his closet...a symbol of the deep and eternal love he held for Ennis...a small and extremely personal way of showing how he had always wanted to 'protect' Ennis... (Only the two of them, fully realizing and understanding the meaning of two innocent shirts hanging together on the same clothes hanger... Can you imagine how lonely it must have been for Jack to have kept this secret for so long??? Twenty years, and not a word of it to nobody... Not even Ennis knew of this...until Jack was gone... :'()
And Ennis, now that Jack is gone, switches the two shirts around, still keeping them on one clothes hanger, and hangs them in his own closet...secretly....probably not mentioning them to anyone...not revealing to a soul, what they mean to him...
Just as Jack did...all those years...
"Jack, I swear..." (that if you were still here, I'd do it differently...) (?)