Upward slope is the default because investors want compensation for tying money up: a term-premium, plus expectations that short rates will drift back toward or above the neutral-rate.
A normally sloped curve is good for anyone who borrows short and lends long, which is most of the banking system. Steepness feeds bank net interest margins and makes carry-fixed-income positive.
Example: 3-month 3.20%, 2-year 3.60%, 10-year 4.20%, 30-year 4.45%. The 2s10s spread is +60 bp, a mildly normal curve.
Related: yield-curve, flat-yield-curve, humped-yield-curve, term-premium, twos-tens