Just as an addition to my last post, for me there are two main thorns in my side which are integral to most of my sites.
The issue as always is speed (page loading times), requests (number of), plugin or snippet location within the page call (header code or body code), compression (minified javascript or compressed or gzipped images, files etc.) and lastly, users browser cacheability.
If a plugin is used on many pages within a site, just an initial JavaScript call can be requested only once and held within the browser cache for faster recall.
These plugins should come with documentation and predefined .htaccess files for includes, expires and cache levels built in to aid the developer to decrease the page load time of any plugin developed.
I find the ubiquitous Google Analytics code snippets and the massively popular addthis.com bookmark php snippet to be very laggy, slowing down so many sites; it's not only unbelievable, but quite intolerable today.