my question is what will be effect of quantum computing on Ethereum?
That's a ridiculously complicated question that can't be answered yet.
Ethereum had to choose a different system and everybody had to agree on it.
Quantum Computing is by all means not the death of any Cryptocoin - Several Universities have written papers about it and there are theories for Quantum Encryption that can not get decrypted.
If you duckduckgo a little about it u will find some very interesting research papers.
: )
P.S: Don't buy Ethereum on Poloniex - I got many many Ether stuck on their exchange and they won't resend it. (Since they don't know yet if it's safe to do so)
Edit: To the guy venting his spleen because of the missing GUI. Doing it in Code is much more convenient. You will get your GUI in the third phase of Ethereum. In addition, Think about it, which Coin ever had 3 Clients available..in different programming languages?! - It's very neat!