Nodes stop trying to initiate connections once they have 15. If you can accept incoming connections, then you can get well above that from nodes connecting to you, otherwise you max out at 15.
I don't know if there's any reason to have 15 connections. Maybe it should be 10.
Since nodes that can only connect out are probably at or near 15 most of the time now, you should level off to an equilibrium. 45 suggests a ratio of 3 out-only nodes to every 1 in-accepting node.
The number of connections won't be a good gauge of the size of the network any more. Someone should periodically IRC to the bitcoin channel on chat.freenode.net and count the number of users. That gives you the total count of network nodes (except TOR nodes).
Block generation is again running ahead of pace. We're in for another big step up in difficulty at the next adjustment in about 5 days.