Ok, that gives me more hope. How do the missions work: you go to the place once and can never go back?
Uh, it depends. All Deus Ex games are essentially linear mission-based FPS/RPGs, but some "missions" are set in hubs where you can explore a fairly large area of a city and do various sidequests within it. Within each such hub, you can come and go as you please. Once you're done there, the story goes on and you can't go back (although you might return later under different circumstances).
Based on what you said, it seems like this is a one-playthrough sort of game. Very low replay value.
fanboy mode activated
The various replies from you in this page is giving me repeated facepalms.
First off, just fucking play Deus Ex 1 at least. It's almost a crime for playing Mass Effect games but not this. Replayability? People replay it a dozen times and still can discover new stuff every time. There's a reason why whenever someone post a screenshot of it, dozens of people would reinstall it immediately.
Secondly, while Deus Ex: HR has smaller areas compared to DX1, mostly due to tech limit (in one of the commentary, the dev stated they hate putting in those decontamination areas to split up maps, but it was necessary because the engine's caching can't handle that), it's still sufficiently large and at least much bigger and more interesting than, say, the largest dungeons in Oblivion/Skyrim or biggest areas in ME. In fact, comparing DX:HR's areas to ME1 and 2 is a blasphemy. In ME you can't jump, sneak, interact with, or even finding a meaningful second-route to finish a mission. All you can do is find cover, pick up ammos and shoot stuff. In all the DX games, each hubs have tons of hidden messages, items, and side quests to find. Tons of NPCs with unique dialogues to listen to. Lots of different approach to one solution. You can smooth talk guards, sneak from a backdoor, or even go full gun blazing. You can kill everyone, tranquilize everyone, or just ghost through the whole game like Thief. And even the game will acknowledge the way you do missions - people will (sarcastically) call you Ghandi if you didn't kill anyone for a mission, or if you go trigger happy in a certain mission, newspaper will tell of a horrible story of someone rampaged through a certain area. Not to mention how much interactivity the areas have: smashing through walls to knock someone out, moving vending machines around to use as cover, making noises to attract patrols and then hit him with a stun rod/gun...
Seriously I don't think we should try to "sell" you the game. The whole series - yes, including DX:IW - is unique and deserve to be one of the best cyberpunk FPS-RPG that have both parts (almost) right. Not playing them (especially DX1), let alone DENOUNCING them, IMO is as stupid as of not playing SS2 and VTMB while calling himself big fan and expert at FPS-RPG.