怎么了is more like asking "what happened?" The connotation of "what's up" is more like “怎么样？”

最近怎么样? It should be noted that "What's up" in American English doesn't have the same meaning as in British English. In Brish English, it usually means "What's wrong" or "What's the matter" ("你怎么啦?"). In American English, however, it can just mean "What's new", "What's happening", or "How are you"("最近怎么样?"), as Taryn explained. There may not be anything wrong when it's used in American English.

In Chinese we usually say : (你)最近怎么样啊？ (你)最近过得好吗？ (你)最近还好吗？ (你)近况如何？ You'd better not leave "最近”out. But I don't think these sentences are slangs. I tried to find a slang in my mind but I failed. Here is a sentence like this "(你)最近混得还好吗？”。Maybe it is closer to slang.

怎么了 或者出什么事了 it is usually used to greeting somebody

I really can't find a sentence in chinese corresponding with "what's up " in English. If we wanna express" how are you", we'd say " 你最近怎么样?" or " 你还好吗？". If we wanna ask "what happened" or " what's up" , we'd say "你怎么了？" And we only raise this question when we feel the people we're talking to is not alright.