There is no equivalent in Eve, except deadspace complexes which have a "DED" rating, where 1 is for frigates, 2 for cruisers, and so on.
Mission difficulty is not obvious either. There is one called 'the Blockade' which is very hard as a level 1 mission, whilst other level 1 missions which are easy. In WoW, each quest has a level which is compared to your own, so orange quests are hard, yellow ones are easy, grey ones are pointless. There's no equivalent in Eve.
For threat assessment you generally just have to know the names of opponents and who poses the most risk, and/or know what each ship looks like to establish its power.
Other clues to the opponent's threat:
- the size of the opponent's little icon in the leftmost of the Overview. Bigger ships have bigger icons.
- the name. There are are elite NPCs. For example a Guristas Infiltrator is a normal frigate. A dire Guristas Infiltrator is an elite frigate with webifying and warp scrambling capability.
- the bounty on the target (how much ISK you get for killing it).
- for missions: the size of the ISK reward - if its a bigger reward than previous missions you've run, it's probably harder; and the time limit for bonus ISK - around four hours or more means it's harder.
- for PvP: the age of the opponent player. Older pilots have more skills, and so generally they'll have more combat capability.
Also note if you're flying a frigate, you're in far greater danger from an NPC in a frigate than an NPC in a cruiser. Your Frigate can orbit the Cruisers at 500 meters and only get hit if the opponent can use a Stasis Webifier or similar technique on you.
My thanks to members of the Eve community who contributed to this information, here.