After some poking around and some questions, here's the impression I came away with.

The place is meant to be used primarily by those who want to make small worlds or to socialize in a way that doesn't involve waiting for clients to install/update, or find 3-D too nauseating. The client is flash-based (which has its own drawbacks, including the fact that the page is always larger than the browser window and right-clicking doesn't work) and loads fairly quickly. Each area is a separate screen, with a limited maximum size of 40x40 tiles (or 20x20 if you don't feel like paying a fee), similar to the long-running massively social Furcadia. The integration of existing databases and search engines for creation of the "worlds" is a nice touch and makes the whole thing more usable for those who don't want to work from scratch, while the customized Lua scripting language is set up in such a way as to provide a heck of a lot of flexibility. The viewpoint can be manually set, as well as a few rendering options, by the world owner, increasing the flexibility of the tile-based system a fair amount.

The biggest drawback, in my opinion, is the limited number of "worlds" you can create. Since there are no multiple floors in Metaplace, the only way you can make a large, multi-story building is to string together a series of worlds with teleportation scripts. The limited number of worlds makes the practical setup of large areas for activities such as gaming extremely difficult and expensive, requiring the cooperation of a large number of people willing to put up the money to keep up large numbers of worlds for each account, and rendering such large projects vulnerable to members of such projects having differences and splitting.

In other words, great for small games, hanging out, and so forth, but not so much for those who want to do large projects. For them, Second Life is still the place to be.