They key traits that Ds1 showed at 2.5 - 3.5 were:
1: Not using language for social communication, or indded to ask for what he needed. Lots of delayed echolalia and he could count and name colours and shapes and letters, but he couldn't say if he was cold or hungry or wet, or what he was doing, for example. At 4.5 he is saying more socially communicative things, but still very delayed and a tad bizarre. Eg he'll say "number 20 right a number 4" to ask to go in some lifts. Or he'll announce something but be unable to put it into context so it'll appear out of the blue, eg "a T turn da light off" or "S shut da door" to refer to incidents that happened hours before he mentions them, or even days before. His receptive language is also significantly lower than his expressive.
2: Very uneven skills. Both him and myself have this, but in opposite ways. He has good non verbal skills, in terms of things like jigsaws and colour matching etc and poor verbal skills. I have good verbal skills (better written down) and very poor non verbal skills.
3: Difficulty understanding emotions and acting accordingly, showing empathy and wanting somebody's reaction to something. We did manage to teach him to cheer and clap when he completed a puzzle which has helped somewhat in seeking other people's reactions to things, but he still prefers to do things on his own or if he wants you it's to use your hand as a tool. However, now, once he has, for example, finished a picture he will tell you what it is if you ask him, whereas at 2.5 to 3.5 you wouldn't get a reply.
4: Adherence to seemingly trivial routines and rituals. Big changes don't seem to phase him (I'm sure they do, but you couldn't tell). Yet little things like his dad getting home from work before he's home from school, his jumper being taken off a different way, not having a blue beaker at breakfast time, not being able to complete part of a routine when walking somewhere, can really throw him.
5: Repetitive behaviour, he will say and do the exact same things over and over. Not completely rigid, but he was and is is rather predictable in a lot of ways.