This is an official list of changes from Sun:

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Solaris 10 Beta Feature Summary:

Storage Technologies NFSv4 - The NFSv4 protocol is supported for both client and server, providing performance and functionality enhancements over NFSv3.

ZFS - (Zettabyte File System) A new storage system which replaces traditional file systems and volume managers in order to provide truly logical filesystems on top of any arrangement of storage devices.

Multipath I/O Boot - Allows booting from MPxIO controlled devices (such as T3 storage arrays).

Solaris Volume Manager - Further ease-of-use enhancements, such as a new tool for volume creation.

Multi-terabyte UFS/SVM System Management Consolidation and Virtualization Technologies Service Management Facility - An expanded mechanism for starting, managing, and monitoring long-running Solaris applications. Services started by this facility benefit from enhanced fault-tolerance, managability, and observability.

Solaris Zones - Zones are isolated, secure application environments hosted by the Solaris kernel. Zones provide improved server consolidation, allowing a single server to look like many.

Resource pools - A flexible configuration mechanism for partitioning of kernel resources among a set of workloads. Resource pools can be dynamically and automatically resized in accordance with system and application goals.

IP Quality of Service - Implements the IETF differentiated Service model enabling the definition of class of service for flows, users, projects, and a mapping to IEEE 802.1p user priorities.

Debugging, Tracing, and ObservabilityTechnologies KMDB (Kernel Modular Debugger) - A more flexible replacement for the existing kernel debugger (kadb), based on the mdb(1M)debugger.

Dtrace - A comprehensive and powerful new tracing facility, which allows dynamic instrumentation of kernel and user programs.

Networking Technologies IPMP Enhancements - Probe-based failure/repair detection is now optional, removing the need for IPMP test addresses and simplifying configuration. The set of drivers that support link up/down notifications has also been greatly extended.

IP Filter - IPFilter provides a stateful packet filter that can be used to provide Network Address Translation (NAT) or firewall services.

Wide Area Network Boot (WAN Boot) - Boot and install Solaris over http and/or https protocols; boot servers connected to the local subnet are not required.

Security Technologies Privilege-based security model - Fine grained control over the privileges with which a process runs. Provides the ability to limit privilege escalation attacks and further secures the system against hackers.

Encryption framework interfaces - A wide range of encryption algorithms are made available to user programs; hardware acceleration is automatically enabled when available.

Solaris x86-Based Platform Support Pentium IV Hyperthreading support SSE (Streaming SIMD Extensions) instruction support

Fault Management FMA - (Fault Management Administration) FMA provides a solution that monitors system behavoir and attempts to avoid downtiime by proactively self-healing a system in any way possible.
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As you can see most of these changes are with an eye to enhance server and resource management/maximisation - Solaris is continuing to drift away from a workstation OS (though I'll doubtless continue to use it as such).

Some of the new features are pretty good. The zones model will let you run an entire server farm on one system, the overhead is very limited unlike e.g. with vmware yet still independent of the 'global' zone.

IPFilter..evolutionary, I already use this on Solaris 9 - saves me a step, if and when I upgrade my firewall.

On x86, the introduciton of SSE and hyperthreadting support is plenty overdue.

Dtrace, very interesting indeed though I've used it but little.

For me, Solaris 10 will not be major introduction but the new applications it will have are very interesting prospects.