First there is a little known struggle over certain parts of 802.11e standard that may require many vendors to change the hardware of their access points in order to comply. That's why there are some pre-standard versions coming out. (BTW: WMM certification from the Wi-Fi Alliance is a complete testing of the parts of the standard that ARE agreed upon). A change of hardware really is undesirable for everyone and could make the adoption of 802.11e a struggle.
Second, 802.11e has basically two flavors - either Enhanced Distributed Coordination Function (EDCF), which basically makes "important" clients' transmissions more "aggressive", or Hybrid Coordination Function (HCF) which supports a mix of deterministic and probabilistic channel access in a sort of "token passing-like" scheme.
In a single Access Point (AP) environment 802.11e will be an excellent enhancement to 802.11. This means that homes, home offices, and small offices will have a noticeable improvement in communication and multi-media applications. Larger deployments, however, will still have problems.
Larger deployments mean many APs close together with many more clients. This severely complicates the last 100 ft problem of the transmission (I generally differentiate between wired problems, or network problems, and 'last 100 ft' problems, or problems with transmission over the air, to and from the AP). When there are many more clients then EDCF has a problem because if there are many "aggressive" clients then there will be (statistically) many collisions because no one is truly wining the random timing lottery - they keep colliding with other "aggressive clients. OK, then lets just use HCF. HCF can help a high number of clients certainly, but in large deployments there are many APs and each AP effectively has its own "token". This is a problem because with many APs it becomes highly likely that neighboring APs will pass their tokens out at the same time and then the clients will interfere with each other. So this doesn't quite work either.
For hot spots and home or small offices, 802.11e will be a welcome addition. For larger enterprises, the only true quality of service comes when there is a network that combines this type of channel control with coordinated transmissions. For a model that uses network controlled and coordinated transmissions you don't need to look very far - cellular telephone networks incorporate this philosophy already. They are extremely large, multi-access point networks with millions of clients!