That's rather not a bug, but I don't really know where else could I post it...

I have watched two games in spectator mode so far. There were stacking items (Leviathan. Mejai's Soulstealer) used in both. The problem is you can't see how many stacks does a champion have.

I haven't seen that, but probable related: If Spectator Mode you can't see buffs unless you see the champion earn the buff.

Example: I logged into the featured match last night at about 3 minutes. Blue side's blue buff was down, no one had died yet, but no champion displayed the buff. Initially I just found this confusing. Later I was watching the game and conversing with a friend over skype. My client froze while his kept moving. I relogged after the Blue team had taken Baron. I couldn't see the Baron Buff on anyone and the only reason I knew they had it is because my friend on skype said so, since his client never froze.

I noticed you can see the Bloodthirster stacking in the item description, but not these 20-stack snowball items.