Maybe I'm being dumb here, but twitter doesn't look like a product from the outside that has many features. Are these focused on advertisers, analytics or what?
We had an API service, a web interface, the legacy web interface that was still used for select devices because the new UI didn't quite work right on them, the even older legacy interface that was necessary because a bunch of badly behaved early day clients still relied on the functionality and they were popular enough that turning them off would cause outrage, the "zero" interface used in countries with low bandwidth capabilities, the mobile interface.
Each interface had to implement all the different variations on functionality. Timelines with inline tweet rendering (automatic expansion of images, etc), list (alternate view time lines), the whole following graph (duplicated for lists as well), verified users and all the infra around that, search, public/private designations, direct messages, notifications via email, text message, and mobile app, favorites, retweets, replies, plus a slew of statistics and information tracking data integrated directly into the site.. Thats only the user visible stuff. There are a TON of experiments and projects that run behind that interface in a way the user will never completely see.
We heard over and over that twitter was so simple that it could run on a laptop and every time it reminded me just how clueless most developers are when it comes to seeing the body of work needed to make something like twitter work, even more so at the scale we are talking about.