Roles are a little misunderstood. Like English grammar, they tend to be more descriptive than prescriptive- they don’t strictly limit a PC’s behavior in combat by themselves. Rather, they sum up the total effects of a PC’s class features and powers, which are focused on- but never strictly limited to- certain patterns. This is where 4e gives you room to have classes that serve the same general role but play in different ways, as well as encourage different builds within a class. A Defender is never just a Defender, a Striker is never just a Striker, etc.
But if we’re looking to build classes and powers, what are the roles giving us in game mechanical terms?
Defenders rely on encouraging enemies to attack them and not their allies. This part is usually done through marks, and each class has a slightly different way of giving marks and of “enforcing” their marks (beyond the rules’ basic -2 to attacks for a marked creature attacking someone else.) Fighters get free attacks of opportunity on targets violating their marks, Paladins get to deal radiant damage, Battleminds deal damage equal to any damage their marked target does to another PC, etc. These abilities give the DM a choice, because there may be situations where risking an attack or damage may be “worth it” to take down a more vulnerable character. (One nice part of 4e’s encounter balance is that it lets the DM play adversarially in combat, knowing that the players have the resources to win.)
Defender powers are almost exclusively melee attacks- a “ranged defender” would likely be a contradiction in terms. Some of these powers reinforce the fighters’ “stickiness”, others dole out large amounts of damage and/or help the defender withstand the assaults of others (letting the defender heal themselves, for example.) Forced movement is also fairly common in defender powers, though it’s often limited to a few spaces. They often make good secondary strikers, dealing out solid amounts of damage to single targets.
Strikers all have as a class feature the ability to deal extra damage to a particular target. The rogue gets the most damaging of these, but is required to have combat advantage- others can be used any time. The idea is that by focusing on the targets of your quarry/sneak attack/etc., you deal more damage than your powers alone would allow you to. Striker powers tend to have strong rider effects, from major debuffing (dazed is a popular status effect to inflict) to ongoing damage to forced movement- the emphasis is on putting opponents at a disadvantage. Strikers usually have equally valid options for ranged and melee builds.
Leaders, by contrast, are focused on giving advantages to their fellow PCs. All of them have some healing ability as a class feature as well as powers which allow PCs to spend healing surges or even heal without surges. When they inflict effects on enemies, the effects are usually ones that make it easier for other PCs to hit them, such as making them automatically grant combat advantage. Leaders are good for creating zones which benefit allies or for creating effects which benefit the team until the end of an encounter. Granting allies extra attacks is also a must. Leader classes are sometimes skewed towards melee or ranged attacks but have alternatives (the Warlord is traditionally a frontline class, but the “lazylord” who spends all their time giving other players attacks is a very popular variant.)
Which brings us to Controllers. Frankly, the Controller role exists in the game because of Wizards, who couldn’t quite be pigeonholed into any other role, and is the vaguest in the game as a result. Controllers tend to focus on two things- crowd control and status effects. Area of effect powers are common, as well as ones that create difficult terrain, walls of fire, i.e. manipulating the battlefield itself. There’s definitely some overlap with the striker as far as status effects go, and Controllers aren’t without powers that attack single targets so it’s easy to see where the two sometimes do the same job. The opposite of the Defender, the Controller works almost entirely at range- it’s easy to think of them as the heavy artillery.
The important thing to remember in all of this is that a class having a certain role does not preclude them from having powers that stray outside that role. The Cleric, for example, is a leader, normally focused on healing and buffing allies, but it can also inflict striker-like effects on enemies and even has a few controller-like terrain manipulation spells. (In this case it’s again tradition that’s partly the reason, since many of the Cleric’s powers are adapted from their old spell list.) In fact most class entries talk about the secondary role many classes can perform- Fighters are primarily Defenders but dish out enough punishment so as not to be terrible backup Strikers, etc.
In creating powers for my own project I’ve tried to focus on creating two separate paths for each class to follow or switch between as they level, with one power geared towards each path. (So my psychic Striker class- modeled after Cronenberg’s Scanners- can alternate between doing horrific damage to an enemy’s nervous system, or controlling their mind and making them act against their best interests.) After the first level you are only choosing one new power when you gain a level where you get powers, so there don’t need to be quite as many per level as there are in the PHBs. But when I get to revisions I’ll probably add a few wild cards that let classes do things outside their role. So long as your class abilities and At-Will powers let you carry out your basic role, it’s okay to not stay entirely inside the lines.