DES (Data Encryption Standard) is weak because of the Key size but what else make DES weak?

1. Key Complement weakness reduces brute force attack to require 2⁵⁵ possibilities.

2. If the possibilities of data what we are encrypting are limited, it's comparatively easier (not easy).

2.1. Differential cryptanalysis: To break the full 16 rounds, differential cryptanalysis requires 2⁴⁹ chosen plaintexts.

2.2. Linear cryptanalysis: FPGA Implementation of the Linear Cryptanalysis was able to return DES key in 12-15 hrs.