Rails 4 now shows you a helpful error when you have pending migrations instead of failing with some weird DB statement error. I like this addition! However, why don't we take a second step and do the migrations automatically?
I'm not sure that I considered all cases but I imagine this should be a safe thing to do in development environment.
First of all, we don't take feature requests on the Issues tracker. This question is better suited for the rails-core mailing list.
Secondly, it'd be irresponsible to just run them automatically, since you can have arbitrary Ruby code in migrations.
not to mention migrations can be "breaking" events which need to be handled with care and planning.