The fast songs of the early albums that make up the signature style of Sonata Arctica sound light and happy like a lot of power metal sounds. Despite songs sounding happy most Sonata Arctica songs are not in major scale but in minor scale. The fast pace of songs makes them uplifting and full of energy. In my opinion the melodies are so catchy it makes the songs sound magical.
The lyrics of Sonata Arctica have influence outside of metal. Many lyrics are about relationships like Mary-Lou is a simple story about a teenage mother who ran away from home. A far cry from other power metal bands singing about sword and sorcery. Some lyrics are dark, darker than it would seem from the music such as The Ruins of my Life.
Tony Kakko has an unique voice: darker and slightly lower than stereotypical power metal singer. I believe his voice type is heldentenor which is a tenor with round timbre close to a baritone. On the first two albums he would sing too high notes that he could not always reproduce live. Tony tried to sing like Timo Kotipelto without considering he would have to sing those songs live on tour. On Winterheart's Guild he moved to sing in the middle of his register where he sounds comfortable. Reckoning Night is the first album with multi-layered vocals in vein of Queen.
On Ecliptica Janne Kivilahti played complex basslines that deserve more credit. For example the chorus of Destruction Preventer has lot of things going on and fits the song such way one does not even notice it. Also the riff at 3:15 in Replica is notable. Later Marko Paasikoski would play mostly root notes of chords playing 8th or 16th notes. Marko's special bassline is the chorus of Land of the Free, best heard in the outro of the song starting at 3:36.
Mikko' Härkin's solos on Silence album are more difficult than Tony's solos on Ecliptica. Many of the Tony's solos are basic scale runs 16th notes while Mikko's solos are have more complex patterns. On Ecliptica Tony did some fast arpeggios though. Mikko practised hard for the Silence album and the following tour. On some Silence tour gigs he even played his solos better than on the album. One of the most the most difficult leads is the harpsichord riff at 2:48 in the song Victoria's Secret which sounds it was played Tony as Jens Johansson's playing style is more legato. Jens' solos on Winterheart's Guild like the intro of The Cage are even more difficult than Mikkos. Henrik Klingenberg's are in turn easier and more melodic. On Reckoning Night there are delicate piano parts played by Henrik for example on the verses of The Boy Who Wanted to Be a Real Puppet.
Many Sonata Arctica songs have VI IV V chord progression. In in C# minor key those chords are C#m A and B which is the key where most songs on Eclipica are in. Broken is different from other early songs as it has strange chord progression.