A few people have explained diminishing returns, but not very clearly. DA and TA are considered to have diminishing returns not because the more you stack, the less your dps, but the more marginal increase in either, the less impact it will have on your dps. For example:
100 rounds
+10 DA (10 total) = 110 hits = 10% increase in dps
+10 DA (20 total) = 120 hits = 9% increase
+10 DA (30 total) = 130 hits = 8.3% increase
100 rounds
+10 TA (10 total) = 120 hits = 20% increase
+10 TA (20 total) = 140 hits = 16% increase
+10 TA (30 total) = 160 hits = 14% increase
etc.
Delay reduction, on the other hand, is a common example of increasing returns.
1000 delay
+50% haste (50% total) = 500 delay = 100% increase in dps
+16% haste (66% total) = 333 delay = 200% increase
+9% haste (75% total) = 250 delay = 300% increase