Macros (not necessary at all, just use regular functions and enable laziness), ints/strings/bools/lists/etc and all their hardcoded features (you can encode all that neatly on the λ-calculus). Every single addition that Scheme makes on top of λ-calculus is harmful in some sense - algorithms on list, for example, don't fuse because of that, making Scheme less performant than the λ-calculus; multi-argument functions make the whole language segmented and are much less convenient than curried functions... etc., etc.

R5RS, the simplest used spec of Scheme, takes dozens of page to merely describe the language. The λ-calculus can be described and implemented in a paragraph. That makes it much easier to create new backends, for example.