Works on Windows, Mac, Linux, Android iOS, Windows Phone, and Firefox OS even. Has a web app that changes it's appearance based on the device it's running on and if it's browser is full screen, etc.

Copy/paste videos, gifs, images, geolocation, and documents, drag over files for practically any time up to 1.5GB in size. Images can be compressed, too.

In Supergroups, anyone along with admins can delete their messages from all sides. A message can be pinned to the top. All of the user's messages can be deleted by an administrator, and they can be banned and reported as well. Supergroups can have a public link (outside of Asia, due to high spam volumes), which is also a username.

Telegram refuses to cooperate with any law enforcement and as a result, was blocked in Russia and Iran. Telegram evaded the ban by using many proxy servers and Russia only managed to cripple its own Internet completely. Telegram worked fine.

Videos, gifs, images, geolocation, and documents.

Very customizable with a lot of clean looking options.

Unfortunately, on the desktop videos don't apply, but images are viewable without visiting popular websites like imgur and Instagram.

You can play YouTube videos and others within the app, so you don't need to embed code or anything.

With inline bots, such as @gif, @pic/bing, @vid/YouTube, etc, you can look things up without leaving the client or chat that you're in. Simply type the name and some keywords and wait for the results to load.

The desktop app has a built in setting to scale the UI of the app up to 200% which allows for those with 4k monitors/screens to use the app without scaling issues.

The application is available free of cost.

You can set up an additional password to your account, which can be reset by your email. You don't have to use an email, if you do not want to. If you lose access to your email or forget your password, you will be able to reset it via text message if your account is over seven days old and will receive it within a week (in case you recover anything before then).

As an open source application, anyone can develop their own clients, such as PlusMessenger (Android), Franz (Linux, Windows) and many others.

Telegram uses an open source MTProto encryption protocol that provides complete end-to-end encryption for secret chats. To show off their confidence in the protocol the company behind Telegram has organized $200,000 and $300,000 challenges to break the encryption. So far there have been no winners.

Uses the latest GPL audio codecs, such as Opus, an adaptative high-quality codec that supersedes AAC at any bitrate.

You can write messages to yourself. You can write some notes, add links, and see everything across all your devices.

Through bots like @gamebot or @gamee, you can play many games in Telegram, alone or with friends.

Whatsapp, being the biggest messenger, is far behind in innovation and feels the need to copy nearly all features from Telegram. Therefore, Whatsapp's new features are usually available in Telegram at least six months in advance. The only exceptions from this are starres messages and photo albums.

There are native desktop, mobile and web client apps, that are optimized for speed and are light on memory usage.

The founder Pavel Durov was exiled from Russia after refusing to cooperate with the Kreml to stamp the opposition and oppress the people. He nearly lost everything he worked on and started Telegram in exile. He has been a big proponent of free speech and unquestioned privacy since then.
https://www.bloomberg.com/news/articles/2017-12-12/cryptic-russian-crusader-says-his-5-billion-app-can-t-be-bought

You can ping people to get their attention by @mentioning them.

In addition to the end-to-end encryption that provides privacy to communications, the app also includes self destructing timers for chats, and the ability to permanently delete messages, yours and theirs, from your device.

Although it's volunteer based, but still pretty top-notch if available, which it tends to be.

Broadcasting channels suitable for announcing things.

Every user can upload as much as they want. The limit for one File is 1.5GB

Even though encryption used for cloud-based storage (e.g. Gmail, Facebook, iCloud) may not be as secure as end-to-end encryption or zero-knowledge storage, Telegram does the most that it can by holding keys to chats in different jurisdictions, requiring intergovernmental cooperation to obtain the keys. This is aside from whatever other security measures have been taken, of course.

Client apps are open source, making whole service more trustworthy.

You can post voice and video messages if you are too lazy to write text messages.

You can set up an additional password to your account, which can be reset by your email. You don't have to use an email if you do not want to. If you lose access to your email or forget your password, you will be able to reset it via text message if your account is over seven days old and will receive it within a week (in case you recover anything before then).