Now on Desktop Contribute Page, I can donate only using PayPal. Will I be able to donate in the future, using, for example, VISA credit card?
We definitely will add other ways to contribute in the future, for this release we only had time to integrate one system and there was an existing mechanism that used to be in our /community section that used PayPal and had an account ready to go, so it was, by far easiest to just use that initially.
As other have mentioned, you can just use use a credit card on PayPal, you don't need to setup an account.
You can indeed donate with your credit card with Paypal, so that might have been a bad example.
The ability to use Paypal gives you a range of options if you have a Paypal account, including credit from items that you sell (such as Ebay items) and even linked bank accounts. It's one of the more secure ways to do business online because only Paypal gets to see your card or account details; it significantly reduces the risk.
If you are looking for other financial ways to support Ubuntu and Canonical, you might like to have a look at some of the great Ubuntu merchandise available: