"Well-liked"
To Willy, the single most important thing in life is to be well-liked. He holds up the example of Dave Singleman (note the irony of the name) as a man who made a fortune simply through being well-liked. The indiscriminate nature of Willy's craving for respect and friendship shows the insubstantiality of his value system. When Charley tries to tell him that being liked by loved ones is more important than the blanket popularity that Willy lusts for, Willy ignores him. Well-liked is Miller's way of pointing a finger at the American Dream: surely success should be based on talent and hard work rather than shallow popularity.
Debt
Miller is exploring the plight of the empty-nester. The boys have grown up and left the house and, in Willy's words:
"Figure it out. Work a lifetime to pay off a house. You finally own it, and there's nobody to live in it".
Willy exaggerates in his estimations of his earnings when speaking to Linda, but even to her it is obvious that they are in financial difficulties. It is in Willy's relationship to money that we see the real tragedy of the man, since it represents the pinnacle of all his aspirations - yet he can't earn enough to properly support his wife. When Charley gives him money, he claims: "I'm keeping strict accounts".
The final irony is played out by Willy's graveside, where we are told that the last mortgage payment had been paid and they finally owned their own house - the apotheosis of the American Dream.