In IPA notation, it's the difference between [ʎɵ] and [ʎjɵ], and contrasting the two is fairly rare in natural languages; East Slavic is somewhat unusual in that regard. If someone's language does not have this contrast, it sounds very similar to them, and distinguishing the two can be very difficult. Even in languages where such a distinction exists, there's a tendency towards a merger - Serbo-Croatian is one example of that, but the same is also happening in e.g. some dialects of Spanish with "ñ" [ɲ] vs "ny" [nj]. English speakers also have this problem with Spanish, by the way, hence why "cañon" became "canyon".
In general, what's perceived as "very different" or not is very subjective based on what one is used to. E.g. the distinction between "v" and "w" is very significant in English, but for speakers of many Slavic languages, those are allophones, and when they learn English they have trouble using them correctly.
True, same for Chinese and their tones. One would think it should be pretty easy to distinguish them, but even when doing basic homework it sometimes hard to tell one from another.
Russian has terminal de-voicing; so /d/ softens to a /t/, hence сад = /sat/. (Sort of; actually it's something in between, but the shift is noticable).
And /l/ palatizes (becomes /lj/) before both е and ё, as in самолёт. (Actually the /j/ is built into ё of course, but somehow it seems helpful to recognize the commonality of the sounds when realized after /l/ - basically е becomes more yoff-like).
The vowel might differ (one may be more fronted or rounded than the other), but that tends to vary among speakers anyway.
Russian has a whole suite of secondary rules like this.
Ukrainian by contrast is much more phonetic. But unlike English, at least Russian has a system.
But they are very different, no?) I'd think mistaking "лёд" for "йод" is easier.