The Datacenter edition is designed for "large-scale virtualization of workloads that require the highest levels of scalability, reliability, and availability to support large, mission-critical applications." http://www.microsoft.com/en-us/server-cloud/windows-server/2008-r2-datacenter.aspx
If this describes your needs then you should use Datacenter.
Virtualization itself will run just fine on Standard, and based on your description it appears you're running tests and labs and such. I'd stick with Standard unless you have a specific need to learn and understand Datacenter.
I don't know, and for a very precise answer, this question many need to be moved to Server Fault, which is geared towards sysadmins and people who would understand the differences.
I'd imagine, though, that the differencse have to do with how the OS prioritizes different processes and systems. A prioritization structure that is best for file and application servers may not be best for virtualization servers, which may, in turn, not be best for SQL servers.
Considering the Datacenter edition can be configured as a Core install, though, seems to me to indicate it is less about out-of-box resource consumption. Installing the two side-by-side in clean states may not appear to be very different.
Regarding whether or not MS has more services set to start by default in one or the other: I've had a little experience with Standard, and very little unnecessary stuff is installed. It's a pretty light (by MS standards) OS in it's clean state. And a version designed for high-availability, high-load systems would seem to me to put a premium on minimizing the default footprint even more so.
It also appears to be, as in the difference between Enterprise and Standard, to be about licensing. Datacenter version includes "unlimited virtualization rights", which isn't about differences in code so much as differences in licensing.
UPDATE3: Sum for the goal...
The differences are primarily licensing-related. Datacenter allows more system resources with higher limits in RAM and physical CPUs (it also requires more physical CPUs initially, beginning at 8).
All versions of Server 2008 install very little beyond the base OS by default, requiring the addition or Roles and Features based on the target purpose of the system.
While I would hesitate to say Datacenter is "better" in licensing terms, it all depends on what you need.
For example, most of my servers (all virtualized via VMWare running on ESXi hosts) need to handle between 1 and 8 cores, and between 4 and 32GB memory. This puts me well below the base requirements of Datacenter. So for me, the Standard and Enterprise versions are "better". I run a few Enterprise licenses and a few Standard, just because that's how they were purchased in our MS Licensing Agreement.
If you've got a system with loads of CPUs and scads of memory and you need to run huge apps on a very high-availability system or run VMs based on the MS Hypervisor, you may find the Datacenter version is "better" for you.
Going back to the very first sentence of my answer: It depends. Always.
I found out the hard way that Standard doesn't support ADFS. In v. 2012, MS is changing this (along with eliminating Enterprise V.) so anyway for 2008, these features are not in Std:
There are a variety of new features in Windows Server 2012 Standard edition. Here are just a few examples of what was previously only available in the premium editions:
Windows Server Failover Clustering
BranchCache Hosted Cache Server
Active Directory Federated Services
Additional Active Directory Certificate Services capabilities
Distributed File Services (support for more than 1 DFS root)
DFS-R Cross-File Replication