UA's check-in lines have been heavy the times I have been there (usually Sunday or Monday morning). But they do have EasyCheckin Machines which are usually empty.

SEA also has a "no shoes, yes service" policy, but then my shoes set off the detector so I have to take em off, anyway.

I have not noticed BWI security pulling people out of line for close flights because I have never experienced delays (in about a dozen trips) so long (more then 15 minutes) that needed people to be pulled if they arrived within the 45-60 min mandated time.