All that host-based card emulation (HCE) does is it enables app authors to support more hardware: that is, devices that don't have the necessary secure element hardware. Developers of apps that previously made contactless transactions using the secure element will now be able to make their apps work on devices that don't have that hardware. However, using HCE is less secure than using a hardware secure element, so it's likely that payment card operators will choose not to support this.
Secure payment cards
The obstacle that stops you reading your own cards into your phone (cloning the cards) is not related to the phone's technology: it's because the card is designed to prevent this. The card contains a secret number that can't be accessed via an NFC transaction. To be more precise, there's no sanctioned way to do this: security researchers and criminals have found ways to use flaws in the design to clone various contactless payment cards. I wouldn't recommend doing this, as doing so may constitute a criminal offence where you are, and will almost certainly make you liable for your bank's losses should anything go wrong.
In summary: the only difference that HCE makes is that if your bank already offers an app to do this, but it doesn't work on your phone, it might (if your bank chooses) work after you upgrade to Android 4.4.
Door cards, etc.
Unlike the high-security payment cards, with processors and secret numbers, door entry cards and tags and the like often just have an ID, which they present to the card reader. Such cards are already very easy to clone. The main obstacle has been getting the the phone to present the desired ID, and HCE means there may well be an explosion of apps that are made for just this use case. (See also Can I clone my (mifare classic) Access card with an Android device?.) However, the long-term result of this is likely to be that door entry systems are changed to use challenge-response protocols and thus become harder to clone.
