As DM:
No controller is easy enough to deal with. No change necessary.
No striker is actually also amazingly simple to deal with, you chop a few hit points off monsters and you're good to go.
No defender is also not a big problem, but the number of encounters a party can deal with per day decreases significantly. Give everyone free durable and you're probably good to go.
No leader is the toughest to deal with. Encounters become very swingy without the ability to have a fallen PC rejoin the fight, especially in small parties. I'd want to always see a leader in a party.
As Player:
No controller is easy enough to deal with, everyone has controllery powers and between race, powers, and items, you can always have some minion control too.
No striker is easy to deal with, you just have a couple PC's lean toward the striker secondary role. Fighters, Wardens, Wizards, Druids, Runepriests, etc. have an easy enough time with this.
No defender makes it difficult to play certain kinds of support characters, narrowing the possible build options. A barbarian, a ranger, a monk, a runepriest, etc. can take point in a defenderless party, but the party won't have many good ways to avoid focused fire, which will burden the leader and tax the healing surges of one or two characters. I've been in a number of defenderless parties and fights always seem very chaotic to me without a defender, and it's hard to funnel the damage taken to the right characters.
No leader is again the toughest to deal with. If you go with an even split of defenders and strikers with a couple dwarves, and a multiclass or two for emergency healing, and maybe a secondary leader role, it can be manageable. For instance a party of 4 with Goliath Battlerager Fighter (MC Warlord), Dwarf Earth Warden, Dwarf Thaneborn Barbarian, Elf Archer Ranger (MC Cleric) could hold their own pretty well. But lack of leader does become a party design challenge.