Why the change? Performance mainly.
But the performance has been poor at best. Even with WordPress caching plugins installed, page renders averaged 3 seconds, not including the fairly frequent spikes (of up to 30 seconds) which would occur every 5 attempts or so.
And while they seem to have more constant outages (two of which have affected me already), the overall page render performance at DreamHost has been at least twice as good (< 1 second for page render on average) as MediaTemple and far more consistent (i.e: no spikes). For those who are interested, without caching plugins installed, the difference was far more pronounced (averages of 13 seconds for MT and 2 seconds for DH).
The DreamHost control panel isn't quite as minimal or as gorgeous, but it is much better organised than others I tried, and has all the functionality I require.
I should note here that these performance results (especially the DreamHost results) are based on a limited period of testing. So I’ll keep you posted as to the service and performance longer term. But it is safe to say that the performance of my previous provider has been less-than-ideal for a long time, hence the move.
For those interested in WordPress performance, I’ll post a follow up about the plugins I’m using.