Class Mechanics - or 'what can this class do and when can it do it' - largely define how the class plays, its strengths and its counters. When evaluating how 'Heroic' Deathknights are in this respect I tend to look at:
How New and Unique are the Class-defining spells and effects? Of the current (pre-WotLK) available mechanics (snare/silence/interrupt etc) how many do Blizzard build into the class. Both Base-line and Talented? How well do the class mechanics generate a unique flavour for the class?
All the analysis is very much based upon the current Alpha build (as of today), and hence is subject to change.
Presences are similar to Warrior Stances in that they are mutually exclusive, active on their own GCD, and self-buffs. Each Presence has a targeted role but are more generally multi-purpose than Warrior stances, and at this stage don't actively restrict abilities. They also have minor aura effects deep in each respective Talent tree. These Presences feature 2 unique mechanics - Scaling Life Leech (4-6% of damage you deal to health) and Global Cooldown reduction on all abilities, both at baseline. Passive Life Leech of this type was previously the province of talented Shadowpriests, and GCD reduction only achievable for casters through the Haste stat, and not at all for melee.
Compared to their most comparable analogues in the Warrior and Paladin classes Presences are more powerful and general purpose. In terms of group utility they lose out to Paladin Aura's unless talented, at which time they lose flexibility but tend to be more powerful. Managing Presences will probably be a large part of DK PvP, and their aura effect will likely be a major selling point in terms of group utility, at least for Blood DK's who tend to lack the amount of group utility of Unholy and tend to be off-tanks.
Strike and Disease System
Deathknights have a huge number of strikes to choose from to apply and exploit their Disease debuff mechanic. This mechanic is similar to the beta Paladin Crusader Strike/Holy Strike combo only much more varied in effect beyond damage. Quite obviously their offense is designed to be limited by their Combat Power systems and so talents which increase the rate of Runic Power (RP) generation and Rune activation will be at a premium. This mechanic also allows for a number of flexible 51/XX/XX builds given that Blood generally caters for Strikes and the other two are more Disease/Caster/Utility orientated. Also worth considering will be how well 2 or more DK's interact and the impact on the Debuff limit currently in place. I think that the Paladin offensive mechanic would look very similar to this at lvl80 if it had evolved out of the Beta system (replacing Diseases with low-power Judgement effects). Over-reliance on diseases for PvP however make Priests and Shaman soft-counters to a substantial proportion of their offense through periodic cleansing effects.
Other effects available to the DK's through their Strike system are a reduction of maximum health by 20% or 3000HP (whichever is less, may work practically as a pseudo-MS), Spell Interrupt and chance to Freeze. So really, there is not a lot of effects denied to them at this stage.
Offensive Spells and Effects
Deathknights however aren't reliant on Melee Combat alone. Perhaps more than any other class they appear to be a true Spell and Melee hybrid (though no doubt Shaman would have something to say about that... Paladins probably wouldn't argue). Among the offensive mechanics and effects they will have at their disposal, talent dependant, are:
Direct damage though ranged spells Ranged Snares Silence Anti-haste (slowing other players casting and melee) Blade Flurry Knock-back/Pull-Forward Area of Effect Damage with 'cower in fear' effect Non-controllable combat Pets and self-rez into Ghouls AoE Repentence-effect (classed as a 'freeze' rather than incapacitate, but amounts to the same thing)
I don't believe that's an exhaustive list. Again plenty of mechanics available there to utilise, many many more than any other class saving perhaps Warlocks. It should be noted, as usual, that this is Alpha and no doubt some of those abilities will be refined and limited in application. Their usage is also highly spec dependant wrt both Talents and choice of Runes - no one Deathknight will have all of those abilities listed, though they may have the vast majority of them esp. if spec'd Frost/Unholy.
The upshot of this is they can be unpredictable in PvP and fulfil gimmicky functions in PvE (within the role of DPS/Tank). It's tough to say whether they will have the raw offensive power of a Warrior/Rogue but they should stack up well against Paladins and Shaman.
Defensive Spells and Effects
Along with a wide array of Offensive abilities, the Deathknight has a large number of defensive skills to take advantage of. Like most tanks they can taunt (and pull the mob to them) and have various oh-shit buttons (such as 40% damage reduction on the next 4 attacks). In addition they have means of gaining stun immunity, and become an Undead for a small length of time (on a loooooong cooldown) with all the benefits and downsides that entails. Unique amongst Tanks they have a number of directed viable self-heals, and like warriors have a number of anti-magic skills including an area-of-effect anti-magic bubble. Finally amongst active skills they have a buff/debuff which causes 5% of healing done to the target to also heal the party.
Many of these effects are unique to the class (thus far), but also from a Lore perspective are strong Undead NPC abilities (e.g. Bone Shield, Anti-magic Shield) which provide the Deathknight with much-needed flavour.
Other Talent Effects
Finally, Deathknights have a number of Passive effects which are granted through their talent trees. There's not much point listing them here, more than most other features of a class Talent Trees are in a great state of flux in these stages. Instead, here are some unique mechanics in the frame currently:
AP scaling with Armour. Avoidance based upon Parry primarily, scaling with AP. Passive Boosts when out of Runes Passive Armour-based 'Oh-shit' (similar to Ardent Defender, scaling Armour rather than DR) +% Magic Damage to target debuffing
This provides the Deathknight with a number of raid-debuffing/DPS/scaling tanking abilities, in essence much more flexibility than even Warriors gain in PvE.
So, are the Deathknights Class Mechanics fitting for a Hero class? Well, at this stage they are certainly unique, flexible, powerful and wide-ranging. They are also baseline in a huge number of cases but within a class that also has huge durability (... tempted to say more durability than Paladins, perhaps even as much as Warriors). At this stage this has to indicate that the class is worthy of being called 'Heroic'.
Oh, and perhaps I forgot to mention... all damage dealing abilities scale with Attack Power, yep even their spells. Food for thought for all those Prot and Ret Paladins around, waiting on Alpha information.