The secound D&D gameday senario was 5 level 1 PC's verses 1 white dragon (2 fighters, 1 cleric, 1 rogue, 1 wizard all pregenereated). Open range, dragon could fly around etc. It was tough, and several people yo-yoed a bit, but we drove it off (was within 1 hit of dying) This was with the DM interpreting "slowed" as you can only take one action per turn. If he'd actually known the rules better (not his fault) we'd have likely gotten in many more actions and killed it a lot better. It was exhasting for level 1 characters, espeacially since it was everyone's first day with 4e, but we won.
So i don't find dragons to be too tough. Plenty tough, but not unbeatable.
Now if you Hover out of melee reach, and just wait until your breath weapon recharges and blast the PC's, yeah, you might slowly whittle down a ranged light party with little they can do. But its going to be pretty boring for every one. You can make powerful use of flight and hover without making it unfun/impossible for players.
So i don't find dragons to be too tough. Plenty tough, but not unbeatable.
Now if you Hover out of melee reach, and just wait until your breath weapon recharges and blast the PC's, yeah, you might slowly whittle down a ranged light party with little they can do. But its going to be pretty boring for every one. You can make powerful use of flight and hover without making it unfun/impossible for players.