It doesn't have a standard spoiler syntax for comments or a spoiler tag(people hack it in with nsfw tags styled to say spoiler but from the main page it will look like porn(ie be marked nsfw)).
It lacks a wysiwyg editor by default. It would be nice to have a nsfw subreddit namespace so that sex related subreddits don't take up possibly safe/useful names and so that you know that /r/humanporn isn't porn. Also a nsfl tag. Ability to expand "ALL" comments. Ability to pick percentage of home page each of your subscribed subs get.
Also a policy that allows removing bad mods from power, ie /r/Holocaust really should not be run by anti-semitic holocaust deniers.
Oh a way to sort posts by reverse age and allow posts in between time x and y not just last hour/day/month/year.
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Of course reddit does have good mobile apps (at least on android), a competitor would need a good mobile app for at least 2 systems which can take a while.
It's having the issue a lot of 'free speech' centric services have; it appeals to a minority who feel censored, but not the mass market that simply want to discuss less controversial topics.
And until there's enough people able to talk about a variety of subjects, the site is basically pigeonholed as the 'place people banned from Reddit might go'.
Reddit, the software platform is open source: https://github.com/reddit
Essentially for a coup of Reddit to occur you have to take a large chunk of Reddit at once. That is very hard to do.