I played an eladrin tactical warlord multiclassed swordmage with the Spiral Tactician paragon path (from Martial Power) in a paragon campaign (from 11th to 20th level) and I loved it. It had just the right mix of support and offensive capability for me. Of course, YMMV.
Some highlights:
To be frank, the swordmage multiclass boosted the offensive capabilities of the character quite significantly. Eladrin Swordmage Advance (from Arcane Power) allows you to make a melee basic attack as a free action against an enemy that you fey step adjacent to. This is boosted by the Spiral Tactician ability to regain fey step when you spend an action point. At 13th level, you can use a power swap feat to get hypnotic swordplay (from Arcane Power), which allows you to stun an enemy for one round, and at 16th level, you can use another power swap feat to get borrowed confidence (from Arcane Power), an encounter utility which allows you to roll twice whenever you make an attack roll or saving throw and use either result until the start of your next turn. If you fey step and spend an action point, that's three (or more - if you use a multiattack power) times you can make double attack rolls in a round.
However, the character also had access to some offensive abilities as a warlord. One of my favorite powers is provoke overextension (Warlord encounter 7, from Martial Power) which allows you to work very well with the party defender. The target you hit is forced to make a basic attack against you, which triggers the defender's mark. Then, if the enemy's attack misses, an ally (usually the defender, or the striker if he is also in range) may make a melee basic attack against the enemy. With good rolls, the enemy may be hit up to three times with a single use of the power: once for the initial hit, once for the defender's mark, and one more time if the enemy's attack missed. This tactic was extra effective in my specific case since the defender was a Champion of Order paladin, so when an enemy attacks me, it takes automatic damage from divine challenge and an opportunity attack from the paladin as well. Apart from that, the Spiral Tactician's fey step assault (Spiral Tactician encounter 11), which is an accurate Weapon attack that targets Reflex, and thunderous fury (warlord encounter 17), which dazes an enemy and (for a Tactical warlord) grants an attack bonus against it to your allies, are also good offensive powers.
A tactical warlord really shines when it comes to giving out attack bonuses. Apart from thunderous fury (mentioned earlier), allies get a bonus equal to half your Intelligence bonus (your full Intelligence bonus at 16th level with the Spiral Tactician paragon path) on attack rolls when they spend an action point. You can also boost damage by taking the Tactical Assault feat. And of course, once per day (or twice per day if you have salve of power from Adventurer's Vault) you can use lead the attack. With unintended feint (warlord daily utility 10, from Martial Power) you can also allow an ally to reroll a missed attack once per day.
In addition to attack bonuses, eladrin tactical warlords can enhance a party's mobility considerably. The Spiral Tactician encounter utility convey ally allows you to teleport an ally, the Fey Command feat (from Martial Power) allows an ally to teleport one square before or after an attack that he makes when using an action point, and the paragon-level Fey Tactics feat (from Martial Power) allows you to teleport an ally whenever you use fey step (this also benefits from the Spiral Tactician ability to regain fey step). In addition, opening shove (warlord at-will 1, from Martial Power) allows an ally to shift a number of squares equal to your Intelligence bonus (or make a melee basic attack) if it hits (and it is likely to, since it is a Weapon attack against Reflex).
Of course, as others have mentioned, try to ensure that you hit, especially when you're using attacks that grant bonuses! Borrowed confidence (mentioned earlier) helps a lot, and try to get hold of a luck blade or couters of second chances (both from Adventurer's Vault) if you can.