What's the difference between AES and earlier block ciphers?
From a user's point of view, AES improves on DES and 3DES in at least two important aspects beyond speed and key length:


AES is a 128-bit block cipher, which makes it more secure for huge volume of data than a 64-bit block cipher is. For example, any 64-bit block cipher in CBC mode is vulnerable to a simple distinguisher attack: the adversary has a chance to recognize plaintext with all identical blocks (e.g. unused portion of a file), because the corresponding ciphertext is more unlikely to contain collisions than it is for random plaintext. That makes 3DES-CBC quite practically unsafe for 32 GiB enciphered; for 8 MiB enciphered, the advantage is already like $2^{-25}$ (fixed).
AES does not have the complementation property of DES (and 3DES), which is that $\forall K, \forall X, \overline{\operatorname{DES}(K,X)}= \operatorname{DES}(\overline K,\overline X)$; that property, while occasionally useful, is undesirable in some protocols, and from a theoretical standpoint makes DES and 3DES trivially distinguishable from a random encryption oracle.