(+) Gamechanging teamfight damage
(+) Win most solo encounters ( carries in 1 combo )
(+) Minion waves cleared in a second
(+) Does a decent amount of damage that scares high MR players
(+) Both armor and magic resist built in
(+) Heal + passive + ultimate + resistances give a lot of survivability when getting towerdived
(+) Forget your mana midgame, it's infinite
(+) Everybody fears your ultimate circle, well placement can lure your enemy into traps
(+) Always-on ultimate: CDR+mana regen makes it your farming tool
(+) You choose when you end the game. Your damage confuses the enemy and clearly shows squishy champions in the enemy team (everybody gets hit with the same amount of AoE damage). Your carry can clean up after you.
(+) Easy assists ( range + cooldown ) and killsteals ( controllable of the ultimate bomb )

(-) No boots: Eleisa's miracle + boots is way too expensive
(-) You will get focused
(-) Pretty squishy. Late game is a kamikaze mission ( viable )
(-) Getting stunned before a combo will kill you endgame
(-) Heal gets most of the time countered by ignite
(-) Probably not suited for serious matches, AP is viable
(-) Heal is lategame only viable in teamfights, the passive does more healing
(-) Low damage early game, pretty useless because you need to play passive like a Nasus
(-) You CAN'T get ganked early game. Mana is a real issue then
(-) Less action, you need to approach the game strategically without helping your teammates sometimes
(-) Before the MPen and CDR you can get killed by towerdiving tanks