Well, I have such a regard for Mr Knightley that maybe I have fancied a bit upon his background and "accomplishments"; but I think that with all what Jane Austen tells us of him, or lets him speak, we can guess a lot and with little probability of being wrong.

One can guess that, although the heir to a large estate (the Woodhouses' wealth is described as "second only to Donwell" and Emma as "the heir to thirty thousand pounds", which, if Isabella and her have equal shares (Isabella cannot have less than Emma anyway), values Hartfield and their other riches, at sixty thousand pounds. This makes that Donwell is worth more, it must be very large indeed, though we hear only about its nearest farm. But Mr Knightley is very far from living on a foot of such wealthy landed gentility (if he has, say, an income of four thousands pounds a year, he could afford a chaise and four, etc); not even on a par with the Woodhouses, who, given what is written in Jane Austen's novels (I have not read yet Mansfield Park, in fact) and some elements given in Liza Picard's "Dr Johnson's London", must be spending less than 1500 pounds a year, while earning three thousands.
Why? Well, of course, there is the recent history, as I guess it, of the two families, I hope I will tell it in another message. This recent history may have taught him to do with little money.
Of course too, he did and he does help his brother, although he owes him probably nothing legally. Or, at the beginning, he helped him settling and waiting his business being prosperous enough to maintain himself, and now he "shares".
But, also, he is modest (not in the way of underrating his own opinions, except when humbled by love!), he means to be, and is, more accessible to poorer people — and a carriage just wouldn't do for that... Although wishing not his children to marry, not Robert Martin's ("his rank in society I would alter if I could" — chapter 54) but, say, the Fords' or the Coxes', he is really deeply involved with their matters, and this, on a part, except for the authority. One can note too, the difference between his attitude to farmers (improving Donwell by enriching the farmers and promoting new methods) and that of Edward and Fanny Dashwood in Sense and Sensibility, who want to take anything they can.
One don't know really how sharp is Mr Knightley's brain, just that he owns excellent sense and understanding; but he is a character of such a depth, Jane Austen has given us with him, really an outstanding fictional masculine picture. I am sorry I am unable to tell more about it, because of my poor English, and my weak talents for expressing my thoughts.
And, about love, my sisters, he is able of something really extraordinary. He is able, seing the one he loves (deeply) doing wrong, to tell it her plainly, even fearing the loss of his chances to be loved by her, instead of flattering her or ignoring the offence, as most men would do. And that, by a strong sentiment of "upright justice" — chapter 48.
One can see nothing such in Mr Darcy. Of course, Mr Darcy is nearly ten years younger, and it does matter. What one can see of Mr Darcy, is that he is outstanding in mind sharpness, really open-minded to the right and willing — successfully — to do anything to reach it, with un exceptional steadiness (Mrs Gardiner sees even obstinacy). He is very generous to the poor, but remaining in the scheme mightly master/poor peasants (one can imagine it will improve a bit after his marriage).
Sorry for such a long post, having not even managed to tell all what I wanted to tell...