What features does enabling ACPI 2.0 support in BIOS enable?
ACPI is required for power management to reduce electricity usage and wear-and-tear on the system components. Your motherboard supports the 2.0 revision of the ACPI standard which of course has some enhancements over the original spec, for supported hardware (that board is, but your other devices like CPU, monitor and hard-drive may or may not be—but probably are). The major changes were the addition of 64-bit addressing and device state and multiprocessor support.
So your options are to have power-management or not, and since you can always simply not use it (turn off the options in the power control panel applet), you may as well enable it in the BIOS.
The only reason to disable it is if it is a buggy implementation which ends up causing problems, but neither the motherboard’s manual nor its forums mention any ACPI related issues. The FAQs for it do include one question about ACPI, but that is a specific Vista related issue which is not even an actual problem, just a notice. Therefore, defaulting to disabled is just a curiosity which if really wanted, can be asked.
If you want to learn specifics about the ACPI spec, you can see the changes in Intel’s Technical Update, or a short, general overview at the ACPI site. For real technical details, you can peruse the full specifications.
ACPI is up to version 4.0 at the moment, but 1.0 performs most of the required functions for most purposes. The useful enhancements of later revisions are support for 64-bit, SATA, and PCI-E, but they also include a lot of stuff that most users probably don’t (currently) have or need like MPS, server, ambient-light sensors, user-detection devices, and USB3.
According to TechARP, this option does not enable specifically ACPI 2.0 features, but enables or disables ACPI itself! Unless you want to go without all power management features (and even if not, it doesn't hurt to have them available), you probably want to leave that option on.
Windows 7 absolutely requires ACPI 2.0 for power management.
It is quite puzzling that it was off and that turning it on didn't have a big impact. You should put your computer into sleep and check that it can wake up, which is a common ACPI problem on ASUS.
A possible explanation for the puzzle is that your BIOS does not fully support ACPI 2.0. The latest BIOS version for your model is number 1702, dating from 2010/10/22, and is available from here (in the Download tab select "Windows 7").
If you decide to update the BIOS, please use great caution and ensure that you also have the media required to reinstall your current version. A botched BIOS update can brick the computer and will require intervention by a professional.