Another question that seems to come up a lot is "What's the different between OCS Standard edition and OCS Enterprise edition?" I won't claim to have a comprehensive answer, but here's a good start of things that aren't mentioned in the marketing materials.

Let's start with what Microsoft.com says about the two versions:

Office Communications Server 2007 is offered in two server editions: Standard Edition and Enterprise Edition.

• Office Communications Server 2007 Standard Edition
  Standard Edition requires that primary server components, as well as the database for storing user and conference information, be configured on a single computer. Standard Edition is recommended for organizations that do not require higher availability through clustering.
• Office Communications Server 2007 Enterprise Edition
  Enterprise Edition enables separation of server functionality and data storage to achieve higher capacity and availability. Enterprise Edition is recommended for organizations that that require higher availability through clustering.

Hmmm, that's it? ...not really. There are a lot of other subtle differences. Read on for more info.

• Pools - Standard edition allows for a single pool to be created. Technically it creates a pool, but it doesn't appear as such in the OCS administration tool, just like a server. Enterprise edition allows for multiple pools to be created. Multiple pools allow for more granular policy enforcement (one policy per pool) and the ability to manage different classes of users individually.
• Database location - Standard edition only supports keeping the RTC databases locally on a SQL 2005 Express instance. This is supposed to scale to ~5000 users, however it can create some management issues if you try to consolidate all your databases together or behind a firewall or if a different group manages SQL and OCS or... you get the idea. The Enterprise edition allows you to specify a remote database server. With a few tweaks it can even be the same server as your LCS 2005 database (requires a separate SQL instance).
• High Availability - You can cluster various server roles with Enterprise edition. With Standard edition you're pretty much stuck with a sticky-session HWLB to manage your front ends (edge roles) and nothing on the back end.
• Scalability - Technically Standard edition supports up to 5,000 users. Realistically I'd cap it in the 1,000-1,250 range if you're planning to use the full functionality (Web conference, audio, voip, etc). With Enterprise edition Microsoft supports architectures up to 50,000 users and from personal experience I can say it scales to at least one and half times that with a bit of planning and tweaking.

If you've come across any other major differences let me know and I'll credit you in an upcoming article/book.