Lemmy vs reddit
Federation
The biggest difference is (and will likely always be) that LemmyNet instances are federated using activity pub. This means that posts, content and comments are shared across different activity pub instances.
For example I have seen your post and am commenting from the beehaw instance even though you are using the Lemmy.ml instance.
Lemmy is able to federate with mastodon and pleroma so you will see comments from there occasionally too.
The main benefit of this (IMO) is that users don't get locked in to a website (e.g. facebook, Instagram, reddit) where they will lose content if they leave.
UI
You can change your theme in settings, but there is not a huge amount of variety, mainly it is colour changes.
The lemmy front end is seperate to the back end so it would be possible to create a more reddit like front end if you wanted to. For example nutomic has creates a bulletin board - like front end for Lemmy that looks like this:
There is an android app for Lemmy called jerboa that has a more reddit like UI that you might want to check out (you can find it on f-droid or the play store).
other differences
Text posts and comments on Lemmy are just markdown so you can do things like embed images (as you can see above).
Edit:
Other differences worth mentioning is the difference in content. Lemmy has less content than reddit although I think there is a sustainable number of users currently. More users will join soon when hexbear (a site that uses fork of an older version of Lemmy) migrates to join Lemmy net. It is also likely that some people will migrate from reddit when reddit ipo's.
