How about malware signature updates? The security software companies I spoke to tell me that they treat their free and paid products the same as far as signature updates are concerned, although there may be some under-the-hood differences between their free and paid products (as is the case with Panda's software, for example). And one company, Avast, says that its free product is intended for average users, and that its paid antivirus is for more advanced users.
On the other hand, free products do give you some flexibility. You can augment a free tool's basic security with countless security utilities. For instance, you can start with Avast Free Antivirus, add PCTools's Threatfire Free (which does a good job at bolstering malware detection), toss in one of the many firewalls available and a link-scanning utility to create your own custom security setup. This approach does require you to do your homework, though, and may be more complicated in the long run.
User interfaces are typically as good (or as bad) in free products as they are in their paid counterparts. Avira and Avast, for example, use the same basic interface for their free and paid versions; they just include or leave out certain features and toggles as needed. Panda Cloud Antivirus (a freebie) is not a scaled-down version of the paid Panda Antivirus Pro, but rather a completely different product with a different interface and different internals.
Most of the free products we tested put up identical or nearly identical malware detection scores to the paid varietals put out by the same company. But we did see some subtle differences. One notable example is Panda Cloud Antivirus: The free Cloud Antivirus and Panda's for-pay Antivirus Pro 2011 performed about the same on the signature-based malware detection tests, but Antivirus Pro did a better job in "real world" malware detection tests that help determine how well a product can block brand-new threats.
(Note: Panda recently released version 1.3 of Panda Cloud Antivirus. The company says that version 1.3 should improve its detection of new malware, but the new release didn't come in time for us to test for our roundup. Check back here for future updates.)