Now that we're all fluent in Polish and Lithuanian, I'll contribute to the game with a little Swedish. Something that tends to confuse foreign learners is the sibilants, though they can't compete with the Polish ones :-)

Basically, there are three of them — disregarding the ordinary dental /s/ —, usually transcribed as /ɧ/, /ʂ/, and /ç/.

The last one is pretty straightforward and exhibits only little variation (affricate [cç] in some areas). Phonetically, it's not a German [ç] as in 'ich', but rather a sound intermediate between [ç] and [ʃ], something like [ʃʲ] (or [ʧʲ]), pretty close to (a shortened) Russian <щ>. 

/ɧ/ and /ʂ/ are more interesting. /ɧ/ can be described as a palato-velar fricative: start with an ordinary velar fricative like [x] in Russian Харьков, keep the back of the tongue in the velar position and the tip of the tongue low, but raise the mid-back part of the tongue towards the soft palate — voilà! It is usually labialized too, often only slightly but in some speakers' version the acoustic impression comes close to that of an [f].

The /ʂ/ is generally what it looks like, a voiceless retroflex sibilant, but it often comes closer to a [ʃ], though usually with some 'whistling'.

In some varieties (including mine) [ɧ] is used mainly intially and medially before stressed vowels — sjö 'lake' [ɧøː] and station 'station' [s̪t̪aˈɧʊːn] —, whereas [ʂ] mainly occurs finally and medially before unstressed vowels — kors 'cross' [kʰɔʂː] and duscha 'take a shower' [˜d̪ɵʂːa]. For this reason they could be considered allophones, but there are some exceptions like försöka 'try' [fœˈʂøːkʰa] and berså 'arbour' [bæˈʂoː].

Historically, we can assume the following scenario: the phoneme /ʃ/ (from various consonant groups, like sj, sk, skj, stj) developed two allophones, [ʃ] and [ɧ] with the above distribution. [ʃ] merged with /ʂ/ (< rs), which could neither occur word-initially nor before stressed vowels (due to the Germanic initial stress) except in compounds (för-söka) or in non-Germanic loan-words (berså < French berceau). (/ʂ/ came about in a general development of /r/ + dental into retroflex: /rt/ > /ʈ/, /rd/ > ɖ, /rn/ > /ɳ/, /rl/ > /ɭ/; in standard Swedish the sequence /rl/ is simplified to /l/.)

In southern Sweden the development into retroflexes never took place (probably because they have a uvular /ʀ/ instead of middle/northern /r/); there they have /ʀs/ instead of /ʂ/, and they usually have /ɧ/ (or even uvularized /ɧˣ/) in all positions, i.e. also in words like duscha. 

On the other hand, in the northern parts of Sweden /ʃ/ (or /ɧ/) and /ʂ/ have merged completely into /ʂ/ or /ʃ/.

These are some of the geographical variations these sounds exhibit. But what is most confusing to foreign learners is probably the large degree of social, stylistical, and individual variation these sounds exhibit. In ares that have both /ɧ/ and /ʂ/, /ʂ/ is more formal and often considered snobbish, affected, or effeminate.

But I've probably given you more information than you wanted already. I might post something on the Swedish accents and possible something else as well.

In case someone wonders about the quantities, I should point out that there are two syllable-structures in Swedish: long vowel + short consonant, or short vowel + long consonant. Cf. vit 'white' [viːt̪] vs. vitt 'white (neuter)' [vɪt̪ː]. Exceptions are again provided by compounds: vedtrave 'woodpile' [˜veːt̪ːr̪ɑːvɛ] ([veːd̪] + [˜t̪r̪ɑːvɛ]).

Urban