Further, the same salt for every password opens the door a class of rainbow type attacks that precompute the rounds of the hash containing the salt, then computing the remaining rounds for each list in the dictionary of passwords. When done well, this can reduce huge piles of computational effort required to scan a dictionary or passwords. This same concept is also why hash(password+salt) is really weak, you can precompute a lot of the rounds of a lot of different paswords and then go from there with the known salt.

There are deeper attacks too, but the above are just what can be derived from a bit of reading up on how the SHA family of hashes works.

For a closely related problem/solution, reading up on HMAC is quite enlightening.