What is it that makes a hero good in the offlane?

The reason it's easier is based on the position of your friendly jungle, and where the creeps "meet". The creeps used to meet much closer to the safelane's tower, but like I said this was recently patched to be more balanced. Another reason the hard lane is hard is because if you are equal distance between the tier 1 towers of that lane, the river is behind you,meaning you are more easily ganked by rotating enemy heroes. Because the hard lane is the other teams safe lane, they will often put their core-farming carry hero there, and give him support to protect him. This means there will often be 2 or 3 heroes in that lane making it "harder" for you as the "offlaner". Also the "safelane" can pull the creeps to neutral camps, helping with the creep control, making it harder for the "hardlane".

Based on this info, a good "offlaner" is any hero who can do well in these conditions

The most notable condition is being easily ganked. That's why most offlaners are either very tanky (Tidehunter, Centaur Warrunner), and/or very escapy (Windranger, Mirana). Some offlaners will have summons they can use to disturb the creep equilibrium, or that are used to last hit, harass, and get vision, keeping your hero safe (Nature's Prophet, Lone Druid). Invisibility heroes can also be counted as a form of escapy hero and are also good to offlane (Bounty Hunter, Clinkz, Broodmother).

Another condition to be considered is that the offlaner won't get much farm, but can get a lot of experience. 

By being alone any xp you get wont' be shared with a teammate, so if you can stay close enough to the creeps to get XP without dying you'll gain a lot of levels. So often, offlaners are heroes who can utilize this XP despite not having a ton of gold. These are heroes with strong ultimate abilities for teamfighting like Tidehunter, Centaur Warrunner, Batrider. Other heroes can counter this lack of farm by being the kind of heroes who can go get farm quickly later (especially with the boost of XP), like Bounty Hunter from track kills, or Nature's Prophet from jungling.

Another good aspect of an offlaner is having abilities with long ranges that can be used to get some last hits and/or harass while staying far enough away to be safe, like Windranger Powershot or Clockwerk's rocket flare.

If you use more than one hero in the offlane

That combination of heroes needs to be strong enough to either threaten the enemy and be able to get kills if they make mistakes, and/or be able to survive/save each other from the enemy. Multiple of the heroes described above could share the offlane, but you lose the benefit of not having to share XP while not gaining anything from having a teammate there (if they both can escape for themselves, why do you need each other). So to get use out of having multiple heroes in your offlane, they need to be able to "do more" than either could alone, meaning they can either get farm, get kills, or prevent the enemy from getting farm. All of which are things a solo offlaner will find hard to do alone, but even some skilled offlaners can do alone.

And aggressive tri-lane is where you put 3 heroes in the offlane

This is done when those 3 heroes are capable of killing the enemy/beating them in a brawl and safely secure last hits for the main farming hero of the lane. You also should only do this if the hero who is now alone in your safelane will still be able to get the farm/xp and not die against whatever opponent(s) are in the enemy offlane.

So what makes a good offlaner. Heroes able to solo the lane generally need a combination of these things. 

• Ability to escape kill attempts.
• Able to disturb the creep wave
• Able to trade harass well with supports.

These help the offlaner to not get zoned out of experience range by aggressive support play. Bare minimum if you can safely get exp somehow, you could possibly solo the offlane.

The best offlaners generally have a few more things going for them out of: 

• Use the benefit of fast levels for early impact around the map.
• Able to retreat to the Jungle and be effective there if pushed out of the lane.
• Able to push out the wave from your tower, delaying enemy lineups that want to push it down.
• Able to get some of the creeps/gold out of the lane and use it to help the team,
• Able to disrupt the enemy farm hero's ability to cs.
• Able to beat or even kill the enemy farm hero if left alone in the lane. This ties down enemy supports.

So some popular solo offlane heroes and what's good about them

• Dark Seer

  He can escape kill attempts by surging away.
  He needs solo experience because he comes online at level 11.
  He can retreat to the Jungle if pushed out of lane.
  He can usually get some safe cs using  Ion Shell and build useful team items like Mekansm in a timely manner. He is melee with great health/armour, can trade well with supports.
  He is good at defending his tower.
  He disrupts the enemy carries farm with  Ion Shell - especially if they are melee.
  He can sometimes even kill enemy heroes if he gets a good level advantage by running them down with  Ion Shell.

• Batrider

  His Firefly can be an escape or CS tool, or even protect his tower.
  He can disrupt the enemy farm/is a dangerous solo hero.
  He can retreat to the jungle.

• Nature's Prophet

  He can disrupt the creep wave, block camps, trade with supports, help CS, control the rune, scout, all with his treants. They do a lot.
  He can escape with Sprout /TP out.
  He can show up anywhere on the map to help secure a kill in another lane.
  He can retreat to the Jungle.

• Broodmother

  Her Spin Web give her sustained regen and extra movement speed in lane. If you can win the vision war they're even more effective and she CS effectively
  She needs solo experience to be a dangerous threat in the midgame.
  One of the most dangerous solo heroes, once she has level 6 very few heroes can stay by themselves around her. She soaks up a lot of resources to keep her down freeing up space for your team.

• Legion Commander

  She can easily escape with fast movement and combined purge/HP regen skill.
  If not shut down effectively and allowed to get a Soul Ring, enemy carries get spammed out of the lane with her Overwhelming Odds.
  Needs levels to start snowballing in the midgame with Duel picks. Can retreat to Jungle.

• Tidehunter

  With a couple of points in Kraken shell and his anchor smash, very hard to secure a kill on.
  Gets quick levels on a strong ability in Ravage.
  Can harass enemies and secure some CS with Anchor Smash.
  If he stacks ancients on Dire he can farm them later to catch up with Anchor Smash.

• Doom

  Good HP and able to move very quickly to escape with his Scorched Earth.
  Can get some gold out of the lane with devour.
  If he is allowed to get a good jungle creep like Satyr regen, Wildwing armour, Hill Priest mana regen/heal, he can sustain in lane pretty well.
  Can retreat to the Jungle.
  It's always good to have Doom available early.

• Faceless Void

  He is tanky with Backtrack  and a Poor Man's Shield
  Very high base damage and good animation means he can out CS most opponents if left alone.
  Escape with  Time Walk.
  Has a great ultimate to get early that can set up your team.

• Bounty Hunter

  Avoid enemy Sentries, get a fast level 6 and start roaming the map for pickoffs.
  Good at disrupting an enemy Jungle hero.

• Lich

  Deny the enemy carry EXP and gold which is very strong. Keep the creepwave close to your tower so you can remain safe.
  Weakness: If the enemy lane is good at pushing Lich isn't good at defending his tower because of Sacrifice. So he's often paired with another farm hero for a strong Dual offlane.

• Centaur Warrunner

  Tanky hero, hard to trade damage effectively with him and harrass him away from the creeps if he has a shield and a point in Return. AoE stun helps escape ganks.Once he has a few levels and Tranquil boots, can go do a somewhat effective Jungle.
  Good global lvl 6 Ultimate.

• Clockwerk

  Dangerous solo hero, can secure 1v1 kills with battery assault.
  Escape with Cogs, manipulate creeps with cogs.
  Once he has lvl 6, can roam the map looking for pickoffs.

• Timbersaw

  Needs solo experience to start snowballing through the midgame. Reactive armour negates harass. Once he has a few levels in Timber Chain, can use it to escape. Dangerous solo hero, can kill enemies 1v1. Whirling Death can zone out melee str farm heroes.