Many Twitter users use URL shorteners to, well, shorten URLs. It's not uncommon for URLs to be beastly things, barely human-readable, and often near or in excess of the 140-character Twitter limit by themselves. URL shorteners have been around for a number of years, from the venerable shrinkster.com and TinyURL.com to the more recent (and shorter!) bit.ly, to becoming a white-label service offered by many organizations including nytim.es and goo.gl to the next stage of shortening: http://➡.ws/
As of today, though, any URL longer than 20 characters will automatically be wrapped in the Twitter URL shortener, t.co. The http:// and top-level domain take up 10 characters by themselves, meaning that it would be a short URL indeed to survive t.co-ization.
So who cares about this anyway? Well, part of the issue with any URL shortener is that if the service goes down, so do *all* the links shortened with it until the service is back up. It's one thing if the URL shortener you use goes down - you can always pick another one. Not so if Twitter or t.co go down.
It will also be more difficult to determine where a link goes. While some tools will unshrink the URL, or will even preview the target page, not all of them do. Twitter claims it is making the change to reduce the number of redirects to malware etc. but it's not at all clear that this is any better.
It also brings a certain amount of provenance into play. Since even shortened URLs are reshortened, the only way an organization will be able to prove what link they originally sent will be to do it manually and introduce a complicated process for capturing the original URL and the original Tweet.
Many services also allow for vanity domains, such as bit.ly/wilkinsontwitter, that can be easier to remember for users, much like a 1-800 number that spells something like 1-800-TWITTER. T.co doesn't offer this today.
Finally, and possibly the crux of the matter from Twitter's perspective. URL shorteners are also linked to analytics. Many services offer analytics around when URLs were clicked, referrers, whether links were retweeted, etc. At some point these capabilities will likely be available from Twitter, but it means being locked in to some extent and it could require payment to access those analytics in a consistent way sometime in the future.
I don't know that there is anything to be done about it today; Twitter's developer blog indicates that at some point this will be done for all Twitter links. But this type of walling the garden and locking users into fewer capabilities is not something that is good for Twitter's users. Organizations that use Twitter in an official capacity should be aware of this and start thinking about the ramifications to their campaigns - and their information governance program.