Memory per port is important because, in a statistically multiplexed network, you need enough buffer allow to smooth out when you have more traffic destined to a port in a burst of time than that port can send out its link in the same amount of time. In other words, when you're trying to send 2 1Gb/s bursts down a 1Gb/port for 100ms, you need enough buffer to hold 200ms worth of data, or something's going to get dropped.
In practice, drops are very bad for performance.
Jumbo frames actually make the problem noticeably worse. Most switches are "store and forward," which means that they have to receive an entire frame into a buffer before they can begin to output that frame onto another port. Meaning you need more buffer capacity to handle bigger frames.
Here's a good technical example of how lack of enough buffer on a SOHO-grade switch causes noticeable performance problems:
http://www.lsc-group.phys.uwm.edu/beowulf/nemo/design/SMC_8508T_Performance.html
The SMC 8505T/8508T was one of the first SOHO-grade gigabit Ethernet switches that had jumbo frames and a lot of people (myself included) bought them and recommended them for that. SMC made a silent revision with less buffer memory, and noticeably worse performance when you have jumbo frames. (they then revised it again, and made it yet worse again. Bad SMC. No cookie.)
For home use, neither QoS nor VLANs are needed. Managed is probably not worth it in terms of cost/benefit, but so-called "smart" / "semi-managed" switches might be (avoid the Dell ones though!).
For business use, VLANs and managed are mandatory IMO. QoS at the Ethernet level I still don't see as a small business feature, for many reasons including the skill level needed to set it up right enough to make a difference.
