Define high?

You can set the transaction fee to dynamic with the lowest fee possible and then the electrum will choose what fee to add, but do remember that it will make you transactions low/medium priority and may take hours to confirm. Or just add 10k Satoshi fee/KB and all your transactions will be sent with that and they will be high priority and confirm normally. There is also an option in electrum to manually enter the fee but as I said, low fee may get your transaction stuck with no confirmations for a long time. If you think about it, 10k Satoshi fee/KB is not that high.

I agree with this, Electrum adjusts it so well you should not even worry about fees.
Just go dynamic fees and pull it to lowest. It will not take so much fee.

Even though I use a common address to do transactions, electrum will throw a warning message when I tried to send all coins "Warning: The transaction fees are unusually high".

When I go to the block chain to look through my transactions, I realised my common address has many outputs after I tried to send all coins. My question is: Aren't all transactions at a common address "merged" or "flattened"?   

Please take a look at a sample screen capture:

That is around 8 dollar cents per kb. Do you really think that is that high?

It is more than the recommended 10k satoshi though. Maybe just turn of dynamic fees and put it at 10.000 per kb.

Nope. It does not matter if you receive coins on a different address or not. Each time you receive coins its an input you can use later. If you dont combine the inputs in advance (and pay a fee for doing so in a TX) they will increase the size of the TX in byte which increases the fee.

What you can do - maybe - to lower the fees is the to lower the number of inputs. If you get them by withdrawing from services, consider withdrawing less often.

Probably no. Both Blockchain.info and Electrum uses compressed keys which does help in terms of the size of the inputs. Some service do pay the transaction fees for their customers but they may have rules you have to follow e.g. no transactions related to gambling.

The amount of the fee that is required to get into the next block is determined by the miner, not the wallet.

Some wallets try to figure out what the minimum fee might be, some require you to decide what to pay, and some are set by the developer.

If you use a wallet where the bitcoins are under your full control (such as electrum, multibit, bitcoin core, blockchain.info, etc), then it is up to you to manage the transactions the way you want to.  You can try to make arrangements to have larger amounts of bitcoins paid you you less frequently so that you have less inputs.  You can use high value inputs to "sweep" up some dust on a regular basis without any fee at all and hope that the high value input has enough priority to get confirmed in a reasonable amount of time. You can just send transactions with insufficient fees and hope that some miner (or mining pool) will confirm the transaction for you anyhow.

Another option is to get an account with a service (such as coinbase).  In this case the addresses (and bitcoins) are actually under the control of the service.  They can manage the inputs and sweep them or use them in ways that reduce their transaction costs.  If they are well managed they may pass those savings on to you (for example by paying the transaction fee for you).

The bitcoin developers are about to reduce the transaction fees on the bitcoin network tenfold, thanks to the relatively high value of the digital currency.
Transaction fees are small amounts paid to send bitcoin transactions around the network (think of them like postage stamps) and to get miners to confirm them by including them in a mining block. They're paid in satoshis (tiny amounts of bitcoins), which means that as the price of bitcoin rises, the transaction fees get higher.