I agree that it's hard to know which witnesses are for what changes but I tend to vote for witnesses who have contributed something. For example one of my votes goes to Partiko because I use that app and like it and feel that such projects contribute greatly to the success of Steem. Granted, there are other witnesses I've voted for for other reasons and I probably don't revisit them enough plus I haven't voted for all 30 either...
I get that choice and that is something I have done too.
If they are not contributing to the current conversation and the impact it will have on the community, they might wind up without a community to use their site.