I joined Twitter in 2008 and have gotten a ton of value out of the service despite its many problems. As much as I hate the platformization of the web, I still use Twitter quite a bit, and I continue to find a lot of great links and discussions in my feed.
Nonetheless, this will be the thing that drives me off the service.
I am a huge fan of Tweetbot, and I when I do end up at Twitter’s actual site, I find it to be nearly unusable. I sort of get why Twitter is making this move—third-party apps provide none of the opportunities for advertising placed (and therefor revenue) that Twitter gets when you’re logged in directly via the website—but it’s still a crappy move.