As someone that deals with markup for the majority of their day I've been bouncing between all of these for the past year or so. I've been using Skeleton for awhile and loved the simplicity.
I tried Bootstrap but the semantics are so horrible I couldn't stand using it for more than one project. I love the tools it has but the semantics of the class names are abominable. I've since settled on Foundation as it had the best tools available while maintaining some level of sanity in the class names and semantics.
Then there are various other non-semantic classes: .btn, .pull-left, etc. It seems to me very much a toolkit built with a developer's mind ... in mind.
I like crafting my css by hand, making it easy to read and understand and above all - easy for me to remember so I don't have to crawl through documentation trying to remember if this element is .span4 .span-4 .span_4 or some other permutation I can't remember. I can easily remember .one.column. Its a column and its one.
See I have the complete opposite opinion. I love the spanX semantics and .pull-left etc etc make complete sense to me so I think you might be on the ball with it being made for a developer moreso than a designer.
I tried Bootstrap but the semantics are so horrible I couldn't stand using it for more than one project. I love the tools it has but the semantics of the class names are abominable. I've since settled on Foundation as it had the best tools available while maintaining some level of sanity in the class names and semantics.