England were eliminated at the Super 10 stage on Saturday following a three-run defeat to South Africa in Chittagong - a result that came after an impressive victory over Sri Lanka and a rain-spoiled Duckworth-Lewis loss to New Zealand.
Giles told reporters: "It is pure frustration really. To play so positively with the bat in all three games and come up short was frustrating.
"After leaving Australia we knew we needed to be more aggressive, not lose too many wickets up front and express ourselves better.
"We did all that in the competition but there are still areas where we can improve - such as in the field and with the ball at times."
Next up for England is a dead rubber against the Netherlands on Monday.