1. Learning what the lifebloom tooltip looks like.
2. Looking for the tooltip on the buff bar of your current target.
3. Switching to another target if you see the tooltip on your current target.
4. Then you just dispel any rejuvs and interrupt any cast heals and you've got a pretty helpless resto druid.
It appears that others are just now waking up to the implications of this weakness, as evidenced by this 10+ page thread on the arena junkies druid forum.
I'm not posting this to say druids are garbage, since feral is probably the most OP 1v1 spec in the game right now and boomkins have been significantly improved over last season. I'll be playing boomkin myself and it's a lot of fun and very strong against melee (though still an underdog against most casters). So druids overall are in an ok spot right now in pvp. I'm just noting that, if anyone wonders whether resto druid will be a strong arena spec next season, the answer is almost certainly "no."