They all had their quirks: the problem with Opensim is that it is basically over 10 years old, and does not allow creating really large terrains. You're limited to quite small tiles, and when you cross a border the next one begins to build up, which is quite an unnatural thing to behold, especially if that next tile happens to contain a mountain range.
Blender is a great tool, but it's not really meant to create interactive environments in. Although it does contain a game engine, it is not a very advanced one.
And meshmoon is just too immature at this moment, and also seems to be targeted towards 3D-web stuff like online social events, presenting stuff, etcetera.
Last month I found that Unity, which is one of the major game engines, was made free for personal use. I had never looked into it because I'm not into gaming at all. But now I was getting curious, and when I had a look I was very much impressed. Though it's also the kind of platform you can create one of those mindless chitty-chitty-bang-bang shooting games in (you know, running around like Rambo with an array of huge guns at your disposal), it is also capable of creating beautiful and very life-like environments. It had none of the drawbacks of Opensim: virtually unlimited terrain size, beautifully rendered materials etcetera. Just look on youtube for "Unity 5 demo" and you'll see what I mean.
I created a scene based on the old Valinor height map that was also used in Blender and Opensim (I created it using maps from the Atlas of Middle-Earth), and extended it quite a bit with more or less random mountainous terrain. It's not finished by far, but to give you an impression, here's a picture of the scene - standing on the shore looking towards Taniquetil, and below that is a link to a screencast video:
http://screencast.com/t/w6Mo8RrE
Eventually it should be possible to create a "game" that can be distributed and played on any kind of computer - even mobile systems, though it should be severely optimised in order to make that work. But for use on a regular PC it's entirely possible.