Just some thoughts on the vocalists:
Joutsen has a more powerful voice. Live you can't get around him, while Koivusaari's grunt did not work well, live. It always lacked volume.
However, his voice has qualities that I like. The grunt is deeper, comes more from the stomach (while you hear more throat in Joutsen) and fits the music excellent. Works great on the studio albums. Koivusaari's grunt has a bit of an unhuman quality. Archaic. As if some creature from the past tells his tales. Gives the albums an extra outlandish touch. The sound works so well that I don't even mind that I can't hear well what he says.
Nonetheless, I am perfectly fine with Joutsen in the band. He's a strong frontman and fits well.
edit:
Some more thoughts on the bass playing. What Olli-Pekka Lainen did (co-founder of the band, he left in 2000) is really interesting I think. E.g. on Tales, at times he plays in a percussive manner (it almost sounds as a different instrument), yet it is very melodic and sometimes independent. Like what Markus Grosskopf does in Helloween, he did that with grandeur in a melodic death metal band.