Deluge vs aria2

Built-in search engines allow finding torrents to download right from withing the torrent client. Deluge lacks such functionality.

Super-seeding is an algorithm used to allow downloaders become uploaders quicker. Deluge lacks support for this functionality out of the box. However, the ltconfig plugin can enable this setting: http://forum.deluge-torrent.org/viewtopic.php?f=9&t=42887.

As of writing of this, there has not been a new release in ~1.5 years.

Sequential downloading can be used to stream media files such as video and audio while the download is still in progress. By default, Deluge requires waiting until the complete file is downloaded before it's ready for use.

But it should be mentioned that there exists a plugin: https://github.com/JohnDoee/deluge-streaming for video streaming from Deluge.

While it has been improved a lot in the last years, when it's used in "headless" mode, the python demon still tends to use more resources than expected. It's noticeable especially on devices like the raspberry, and for instance, rtorrent and transmission use much less CPU.

Deluge has minor UI issues such as presenting "delete with data" as the most prominent button when removing a torrent from a list of downloads, or the way it resets its size when maximized from tray forcing a resize each time.

If a magnet URI is used there's no way to get information about contents or perform any kind of actions (such as selecting what to download) before the download has started.

Web seeding support allows downloading file parts from an HTTP source alongside the swarm. It's usually used for long-term seeding and easing some of the load on web hosts that supply direct downloads by offering a BitTorrent download as an alternative while still ensuring that the file is always available in its entirety regardless of the swarm. Deluge lacks this functionality.

Deluge creates all the files even those that you marked "not download". Making your directories messy.
This is very bad when you want to create a drive specific for large files and you are unable to delete or move small and unwanted files, such as a video partition with large allocation unit.
[qbittorrent doesn't do that]

There's no way to manually override settings for download queuing that's available in other torrent clients. For example, if there are 2 files in the download list and the software is configured to queue just one at a time there's no way to force a download of both without changing the settings. A workaround plugin can be found here.

Searching for torrents or files within torrents is difficult because it relies on the user to input the exact title with the right index for each character.

Deluge uses ~10% of one Core i7-2600 core with ~1700 torrents loaded, even if all the torrents are idle.

Tracker exchange functionality allows peers to exchange information about trackers of a given file. It is another way of ensuring that the file is downloaded from the optimal source. Deluge lacks this functionality.

An embedded tracker allows turning the client into a server offering an easy way of sharing files with others. Deluge lacks embedded tracker support so another method of sharing or different client has to be used to share files.

Broadcatching is a method of automatically downloading content as soon as it's available. Broadcatching in BitTorrent clients is usually implemented via RSS. Deluge requires manually selecting each download by default. Fortunately, there are multiple plugins available that add this functionality with the possible caveat of plugins not having the same quality standard as the client itself.

While there is a web-based GUI, there is no native interface.

While the developer of aria2 is developing nghttp2 library, aria2 has no support for http2.