The obvious, the linear is Hard. There is a real chance of failure and/or death. This does not have to be in terms of massive gribblies. The Tree-ghouls linear was against only tree-ghouls who were tough monsters, but nothing stat-wise that we couldn't handle as a party, but took on a whole new dimension of difficulty in the woods of royston. You can run a Drac mode linear easier with a 150XP party than you can at 480XP as the high level party have much more variables that you need to challenge, and thus many more ways to accidently TPK them.
Hard is a very difficult things to judge when statting linears. At the end of the day, if the refs want to TPK, they will TPK the party - a level 5 necromancer with his 3 ghouls and 3 zombie collossi will end any party, causing this accidently is very easy when you are trying to push a party, particularly with CUTTs resource depletion. This is no fun for anyone. The key element to the linear is the linear is unforgiving. With most linears, if the party do not work well, or screw up a few times, there is usually enough margin for error for them to come through. In a DracMode linear, if you screw up, it costs you, usually in body count. Good things for doing this: a variety of monster abilities in each encounter: Warriors are very good at killing lower level warriors, 2 warriors are excellent at killing mages (1 takes the status effect, the other beats the mages head in) having some warriors and some mosnters able to throw status effects changes the battle, as now the monsters can take PC's out of the fight as well.
Given the lack of forgiveness, the DracMode linear should encourage teamwork. The party either pulls together and fights as a unit or dies, and actually is very good for getting things like group cohesion and fighting together into LARPers heads - by forcing it. Things like staying as a group, keeping an eye to see if anyone is doing badly / down / has been hit with a FREEZE become very important. In general, if a group of individuals can win, it is not a DracMode linear. If you PvP? during the linear, its probably not going to end well for you. The Tree ghouls were a wonderful example of this - 1 on 1, they would probably win against most of the PC's, we had to work together to protect each other.
The "Bandit Encounter" is incredibly important in one of these linears, and in my opinion becomes "encounters". This a hold over description I have from warwick were the linears would usually start with a low level combat encounter to gauge the party. Even the best parties have not trained a lot together and so it is rare to find a linear party that immediately gels and works well together (In general most players will linear between 5 and 10 times a year, with different characters and character combinations, this does not give any given party a great deal of experience in working together. The best you usually have at starting is a group of players who often fight together, but even that needs time to adjust to different characters have different ways of fighting). Taking from my above point about needing team work and co-ordination, the linear needs to let the party develop this before it whacks them with the big stick. A few encounters steadily racking up the difficulty are all you need and it gives that small amount of forgiveness at the beginning for everyone to sort themselves out. If you let rip all guns blazing in encounter 1, chances are you will TPK in encounter 1.
The success of these linears should not be critical to the over arching plot. As there should be a good chance of failure, you don't want to be tempted to downstat to make plot happen. Have a success plot outcome and a fail plot outcome and let PC's deal with the consequences.
In general let the characters have a way out, and because players are stupid, let them know at the start what it is. Given the difficulty, it is often a perfectly valid IC choice to say "screw this, we are not going to succeed, let's bug out and not die needlessly". Sometimes there is not one, that's fine, but in that case I specifically draw attention to the OC information bit next.
You should inform people out of character what this is going to be before hand. CUTT is a low risk system and the players are whiny. High chance of death does not appeal to everyone, and those it does not appeal to will let you know about it if they find themselves in it by surprise. They should be aware of what they are getting in to before they go, and an OC "actually i don't want to go on this" taken with good grace and quietly let slide IC, same as you do for someone who can't make the linear due to OC commitments. This is especially true if there is no escape but succeeding - being trapped in something you didn't buy into that will probably cost you a character is likely to upset people.
A side note for that: a lot of the people who it does appeal to have jobs/girls/boys/other larp events that eat into their weekends, if you think the linear is interesting to them, you probably want to let them know in advance so they can pencil you in. I personally think the wiki page with the rough combat/threat/main activity of the terms linears was an excellent idea, because without giving away any major details it let people see what was coming so they could pick their linears to be the ones that suited them.
For reference, I have been on 3 linears where I have explicitly asked the Refs for Hard Mode, across 3 ref teams, and I have died on 2 of them, all 3 have been excellent linears that I have really enjoyed. Both deaths were for entirely preventable reasons - Cal died because the player party dropped him a 2nd time when they revived him after the "final" encounter and he was still berserk, leading to a delay in us leaving and being hit by the 2nd wave of monsters. Glenn and Ari died due to not killing the humacti to maintain their disguise with the azraelites, thus having to deal with all those humacti who have just healed themselves and were pissed, after PvPing? the rest of the party in the final encounter. The 3rd one Skord survived by sticking with the party and supporting them as much as he could, whilst being terrified as he was stuck in a forest filled with viscous monsters that would be able to kill him if he got separated from the party.