That also lets you do a one-field quick task entry that includes +collaborators or an arbitrary number of @tags, Those appear to be the two pieces of data you capture beyond the descriptive text. You get a few more than ten projects too, which is the max number you support for the most expensive plan.
The big obvious difference is the pricing model. You're $80 a month for unlimited users on the annual startup plan, vs. $5/mo/user for Todoist Business. For an org of any size, Todoist would get more expensive for sure.
But under that 16 team member break even point, it sure looks like you're implementing a pretty sharp subset of standard features. And when you consider Todoist Premium is $3/mo/user (so now a 27 person break even) and can be used for simple workgroup collaboration as long as you don't need centralized billing or a team admin interface--which seems squarely like your target based on the simplicity pitch--you're very much on the high side of pricing, but with a minimum of features.
I'm not saying there's no advantage here, but I certainly can't tell what it is from your landing page. Once you're at enough users where the pricing makes sense, I'd think you'd really miss a lot of other standard functionality. Unless there are queries, due dates, the ability to attach a comment chain (or at least a notes field!) to the task, etc, I wouldn't think this would cut it. If those features do exist, they're too well-hidden before signing up.