Avoid Matt's El Rancho like the plague.
My information is from a perspective growing up in Austin, Texas and eating a lot of Mexican and Tex Mex food, and whenever possible I try to sample *-Mex wherever I travel.
I am a bit puzzled where you are getting your information from, since it is not accurate. In fact, it's puzzling where some of the other people in this thread get their information from.
Tex Mex is basically Mexican food plus a lot of creativity, e.g. there is a hamburger joint in town that serves a catfish breaded in tortilla and fried, covered in queso and served with pico de gallo. Tex Mex is a pervasive thing here in Austin and Tex Mex food tends to be fun. Anything goes in Tex Mex, so it's very hard to pin down. There is an emphasis on flavor in Tex Mex, and it does not have to include meat, meat sauces, or tomatoes. Tex Mex tends to have some heat. If I travel outside of Texas, I tend to notice that the levels of heat, spice, and overall savoriness tends to decline, including California. There are pockets of *-Mex around that are close to Tex Mex, but I tend to find out that the people making this food are from Texas.
Friends of mine that have lived in California tend to agree that California Mex is quite a bit more tame and tends to emphasize fish and such more. Some people like the California Mexican more, probably because they are from California or they are not from Texas and so are not used to the amount of spice and heat used in Tex Mex.
In general, it is quite hard to even define exactly what authentic Mexican cuisine is, since Mexico is a very large country, and one time encompassed Texas, California, and New Mexico. It should be no surprise that the variations of *-Mex in these states are different.
Tex Mex rellenos do not use Hatch peppers, they tend to use poblanos. I have eaten a ton of these things.
We have had freshly prepared guacamole since I was a kid here in Austin. We call a tostada a tostada, not a tortilla chip, except when you get served chips and salsa, where the chips are the same as in your usual bag of chips. With some salsa on the side.
Chips and salsa - I'm not sure where this was invented, but it is very common here in Austin. If you go to a Tex Mex establishment, you will generally be able to get chips and salsa while you have a drink and wait for your food. There are usually different kinds of salsas to sample from, most of these salsas are heavily influenced by the Mexican cuisine Tex Mex is derived from. You can also get chips and queso if you desire, or just a plate of nachos. It is not uncommon for the chips and salsa to be complimentary.
Margaritas. We love our margaritas. These aren't Mexican margaritas, they are Tex Mex margaritas. I think the Mexicans would refer to these as Kool-Aid. Anyways, you can get them anywhere, and they are tasty and come in many different flavors and some can get you very drunk, very quickly.
Mexican Martini - it's basically a modified margarita, but usually served up in a shaker and limited to two per patron. There's no gin in this, instead this drink uses Tequila. There's no real established recipe for it, but the recipes tend to be very similar. Very popular around Austin right now. Some snooty bartenders won't serve them, but the demand is too great.
Tacos, tacos, tacos. We are huge on the taco. There has been a taco explosion in Austin lately. I hear that the burrito is the big thing in California. Here you can get breakfast tacos anywhere. I hear it's the breakfast burrito in California. Personally, I favor the taco. Tacos here are usually only rolled when you are getting them to go, otherwise they are usually folded or simply served flat so you can add condiments and then fold them. Tacos here are usually not crispy, but I have been told the crispy tacos are popular in San Antonio.
Breakfast - Just like any other American state, we like our breakfast. So, there are lots of places that serve Huevos Rancheros and Migas here. Most places have an array of Tex-Mex breakfast foods. Migas has gotten to be a staple dish around here (again, Austin). These are actually more Mexican than Tex-Mex but most Tex-Mex places will serve a variation of this (e.g., extra cheese), although it might be just the plain old Mexican version.
Salsa - I don't know what salsa in Cali is like, but I love Tex Mex salsa. There are so many kinds, green, red, smoky, sweet, herby (there is a cilantro salsa in town I love), chunky, not chunky, and so on.